How do I clean a 13” mid-2012 MacBook Pro motherboard
Test it first!!

Okay, this may seem strange (and many would disagree with me) but based on your question it sounds like the MBP was immediately shutdown after the spill and hasn’t been used at all since for about five months. If that’s the case there’s a very real chance the MBP will be fine if there wasn’t too much coffee penetration and the spill wasn’t near the worst possible areas (you don’t say where the spill was, or how much of it there was). But, being human, I assume the first thing your brother-in-law did (besides shutting it down) was to remove the MBP from the coffee spill and soaking up any obvious external signs of coffee with paper towels. This may be wishful thinking on my behalf, but I figure if he had the sound mind to shut it down, unplug it, and not restart it again (plus the knowledge required to remove the hard drive), that he had some idea of what he was doing. 

I guess what I’m saying is, if it was me I would connect it up to a bootable drive (you can just do this via USB) and power it up. You’ll know soon enough if it’s going to work. If so you’ve saved yourself a bit of effort without the risk associated with cleaning a logic board (i.e. motherboard) etc. If it doesn’t work, your chances drop significantly, but not totally. So below is how I’d proceed to clean the logic board/components. 

WARNING!!!

Before you begin ensure the MBP is not connected to an AC power supply. 

Removing the bottom cover

The bottom cover has 10 screws. To remove these you will need a Phillips #00 screwdriver. While you may get away with another method to remove them, don’t do this as you may thread/damage the screws in the process!

Also, I strongly suggest as you remove each screw you place them on a towel or similar material so they don’t move/roll away, and that you do this in the pattern that you removed them. This is because these 10 screws are not actually all the same - three of the rear ones are about 3x longer than the others, and the four front ones are shouldered screws - so make sure you don’t get them mixed up! 

Recommended:  I strongly recommend you follow the first four steps of this iFixit guide - it shows you how to remove the cover and how to also remove the battery connector from its socket on the logic board. 

If for whatever reason you don’t feel comfortable removing the battery connector, you can try to ensure the battery has no charge in it!! When you’re satisfied this is the case, press the Power Button for a while (say 15 or so seconds) and then let the MBP sit there for at least 20 minutes before doing anything else. It’s a good chance to grab a coffee - just make sure you don’t drink it anywhere near the MBP! :)

Tip: It’s also good to use a magnetic screwdriver to remove the screws (just so you don’t drop them on the floor - most are too small to find easily).

Once the cover and battery connector are removed, proceed below.

Cleaning dust and other dry substances

Anything dry, be it dust, the remains of an old insect, etc, should be removed by gently brushing it off with a totally clean brush. You can actually use a small clean paint brush (preferably an artist’s paint brush) or even a toothbrush - just remember to be gentle and not to use anything that’s wet or dirty! 

In some cases the brush is just loosening these substances and if you can’t whisk them away with the brush, you can use a can of compressed air to gently blow them away. 

Notes re: using a can of compressed air - Make sure you do not shake the can or spray it upside down. Before spraying anywhere near your logic board, spray the can elsewhere first to get used to controlling the air pressure.  When you do use it, try your best not to spray it at an angle.

Cleaning grime etc

In this case we know you’ll also need to clean a type of grime (after all, dried coffee is going to be somewhat sticky originally especially if it also had sugar and/or milk in it). Regardless, the process is largely the same for anything that’s a grimy sort of mixture, be it coffee, coke, juice, etc.

Because the inside of a MBP is extremely precise and everything fits in perfectly, even dust can have a hard time getting in there (quite unlike many desktops!). Obviously, cooling fans may collect dust etc, but I would give the logic board a really good looking over because the coffee spill may be limited to only a small portion of the interior, in which case you can limit the cleaning only to the affected area!

If you find that there’s coffee on any of the removable components, then remove those components (e.g. this MBP has two memory slots, so remove any RAM modules). If the RAM modules look like they’ve been affected by the coffee, then you can clean them separately later. Note: If you do remove any components, make a note of where you removed them from. Also take a photo beforehand!

Now, I’ve seen a lot of different cleaning solutions used over the years to clean the insides of a computer, ranging from household detergents that do not contain phosphates to methylated spirits (once I saw it done with a paint thinner). However, my strong recommendation and the only liquid I ever use inside a computer is a rubbing alcohol (such as Isopropyl Alcohol). You can buy different grades of purity, but I prefer 100% Isopropyl Alcohol (or close to it). You should be able to buy this from a good electronics store or hardware store. Otherwise if you get stuck, check your local supermarket for what grade of Isopropyl Alcohol they sell. Even if you can’t get close to 100%, a lower grade of Isopropyl Alcohol is preferable to most other cleaners.

My personal preferences is to pour a little of it onto a lint free cloth or terry-cloth towel or lens cleaning cloth. If you haven’t got one of these, buy yourself a lint free or terry cloth - it will come in handy many times over. If you get really stuck, you can use coffee filters. Either way, use one of these to clean anywhere on the logic board that has signs of coffee on it.  

Now, if you did have to remove any RAM modules because they showed signs of coffee, then start by using compressed air (remember my tips above on using these cans) to blow them clean, especially around the contacts. Now also do the same where the slots are on the logic board. Assuming you did get your hands on some strong Isopropyl Alcohol (say 95% +), use this to dampen the RAM module contacts and then insert the RAM in and out of the slots a few times before proceeding to clean the contacts.

Once you’re done, the Isopropyl Alcohol shouldn’t take long to dry - but just to be safe you may want to let things sit a while before putting it all back together.

Testing it

Once you’re satisfied everything is dry and put back together properly, try powering it up. If you have no joy, take note of any audible cues you get (e.g. startup tones) or whether there’s any sign of it powering up at all. 


If you do get audible cues and/or hear signs of it powering up, then there’s things you can try (such as resetting the SMC, resetting the NVRAM, running Apple Hardware Test, alternating memory modules, replacing certain components, etc), but I’ll leave that for another question if it gets to that point.
If you get absolutely no signs of power you could decide to remove the logic board altogether and inspect it for other areas requiring cleaning. You can view this iFixit Guide to do so, but keep in mind that this is much more involved then the process above.


Notes


If you decide to insert a replacement drive, you can follow this iFixit procedure for replacing the drive.
Remember to keep a track of which screws go where!