sounds to he want to recombination to you

9/10 times they dont work out. your chance will come

That's what happens when you "hook up" with someone without commitment.

While ultimately it is the individual's responsibility to keep their own limits in mind, if the best friend had known OP really was in love with him and casually slept with her anyway, that would be pretty dickish. However I'll give him the benefit of the doubt. It should really be established before sex what the sex means, but spur of the moment does happen, and in those cases, you really need to follow up and talk about what it means, particularly if you have a close relationship with the person. If there is no follow up, you should assume, to be on the safe side for your feelings, that it was nothing more than some casual fun for the both of you. I never used to be constant about following this advice, and since all my hook ups have been casual, and for awhile it worked well. I even hooked up with a friend and there was no weirdness. However one guy(who aside from a few messages and a casual date before hand I didn't really know) got attached, and thought it meant more. I felt pretty bad, because I have had people mess with my feelings before, and I know it's no fun. TL,DR for your sake and theirs, TALK when you hook up. And if you don't talk, assume the most casual of meanings.

You should not hold onto someone who might never have the same feelings that you do, or mistake having sexual intercourse for an emotional reciprocation. In addition, while this person and you are close it is quite tactless to request your opinion about someone else, especially given the history. The hardest, and best decision you can make is to allow this person to leave your life, otherwise you will ache each time you are asked for relationship advice.

Guess you don't know him as well as you thought...

Never EVER hook up with a friend it usually ends bad