Can't get switch and lights to work
Put it back, call the landlord

Two things are true anytime electrical work is done on an apartment.   


The work must be done by an electrician with an actual license. 
The landlord must give permission for the work to be done.  


The landlord doesn't have a choice, the electricians only rule is not waivable.   And any smart landlord catching someone working on electrical would evict them and charge them to have their electrician inspect/fix everything in the apartment. All pre-existing faults would be blamed on your tampering, and what could you say? 

One thing about electricians is they have the tools and know-how to "ring out" wires. They can find out where the wires go, and if they are broken. 

That's the first thing I would think he would do, identify wires and cables.  Not a simple or easy job.  

But this isn't going to work; him working the problem and you looking over his shoulder and asking us. You won't be able to give us the information specific enough to solve the problem, so it will amount to commisseration, and that's better done with friends.   Second, where would this go?  If you look over his shoulder and say to him "try ringing out the travelers", he may have some questions about where that came from.  

Really, the person who is actually working the problem should be asking such questions, and you may have heard this too often in your life but in this case, you are better off leaving that to a licensed electrician owing to the "electricians only" law.