How to train front braking technique?
Lets get the social aspects out the way - you can lead a horse  to water and all that stuff.....: Hows your relationship - will he listen to you and is he prepared to take clearly well intended advice. Also does he believe he has a problem and does he want to fix it. If any of these answers is no there little you can do except ruin you friendship. Till hes ready, wait at the bottom of the hill for him, cut him plenty of slack and make sure you both enjoy the riding - if appropriate laugh with him, but never at him. 

After a night in hospital and metalwork keeping my wrist together, it took years for me to be able to ride a track with large drop off to the side - especially the right - comfortably, yet and off over the handlebars never phased me. After a big off you subconscious needs to be convinced you will not be hurt if it happens again, or it will not happen again, the only way it learns is through repetition without the consequences. You mate has learned hitting the front brake throws you over the bars, so his subconscious will not let him do it.

Presuming hes ready to learn, the and hes is willing to learn from you - Look online for some videos - watch for the rubbish that is out there but you seem to know what you are looking for so should be all right there. 

Chose something he can currently ride comfortably, that you ride faster, with more control and/or more comfortable. Ideal would be a could of tight switchbacks that can only be ridden fast using the front brake to slow into them, yet back only braking merely means you have to go in slower. Safe run out if you don't make it is essential, bonus if its a lake or river that gets you wet or in front of a big crowd of onlookers who will give a good cheer at the screw ups.....The idea is to get something that is safe (physically safe as well mental and ego-safe). He needs to ride something that is not doing in his head, that his subconscious is going "bring it one, this is easy" while his conscious is think how to do it faster/better. 

If possible video you and him riding it on the first run and keep it for later. Point out where you are braking differently and ride it for a half day. Each time, both of you try to go faster/more controller/better. If you get bored or it gets too easy, find another spot just a little bit harder.  The aim is ride technically easy ground perfectly before progressing to harder sections.  

At the end of the session, review the video of the first and last run over a beer.... 

Work out if you are going to mix this in  with some rides, at the end of a ride is good, but make it really easy as your both tired. A special trip is well worth it.