"Working on this together" would probably be the worst possible thing you could do. I've seen it in breakups with me, friends, it's always the same. People want to pull the friends crap while there is too much anger and resentment in the air. I'll say the same thing to you that I do to everyone else, don't call them, don't see them, don't write them, don't email them. Deal with that person on their terms. Even then it probably won't work.