I really need HELP on what to do with about my son!!?

I am married and have 5 boys. They range in age from 23-11 years old. My oldest is married, and has been for about 2 years now. My 19 year old son dated my daughter in law's sister for about 1.5 years and they ended the relationship almost a year ago exactly. He was starting to treat her like crap (and everyone for that matter) and that's when she broke up with him. He was diagnosed with moderate bipolar disorder the following month and once he was on medicine he became his old self and a great person. She wouldn't forgive him for what ever reason, even though it was no fault of his own. He had apologized repeatedly, yet wasn't able to convince her to give him another shot. She started dating his best friend (who he no longer talks to after that) and the best friend started making up lies about my son. The lies were rather severe, almost to the point where my son was going to press slander charges. Well now we are having her family over in about an hour for Thanksgiving (since my oldest son and their oldest daughter are married, and we are technically family). My 19 year old son is extremely pissed at my entire family now, saying that we are all very inconsiderate and that he refuses to spend Thanksgiving in the same house as that b*tch. I take it he's still not over her since she was his first true love, and he lost her under such horrible cirstances to a complete jerk of a guy. I know my son has messed up in his life but he did not deserve this, he treated her like a princess until he started becoming bipolar. So now I am stuck in a position as to what I should do...Any help or thoughts would be immensely appreciated!