Forgive them, so you can move on in life. Dwelling on something so small (at least, that's how I see it) is physically and mentally stressing, unnecessarily so.
I think you should try to work things out with your friend because:
a) Friends should take priority over the people you date. I say friends first because friendship is especially important, at least for me.
b) It is unfair to hate either of them because they decided they had something going on. Other people's feelings, physically based or not, are something that are out of your control (but not necessarily out of your influence). i.e. you can't force them to stop liking each other.
Whatever he was seeing a therapist for, it would probably be beneficial if he did start it up again. If he goes to college, most offer therapy/counseling (not just academic). Although, because of the budget cutbacks (in California) the hours have been significantly reduced, so you should look into the hours and such. I think it sounds like he is displacing his anger about his mother denying him therapy onto you. There are also usually teen helplines that you can call, or suggest he call. I just googled for "teen helpline" and came across this link:
http://www.teenadviceonline.org/gethelp/numbers.html