I know him well, only met her once; she's divorcing him, think grounds of unreasonable behaviour, he wants to counter sue her for same, saying she knew he had MH issues when married, so can't say he's UR now. He does have MH issues, but aside from that he is rather a selfish, immature arse, just because he is one rather than MH related. He's being very unpleasant, talking of hiding assets, making her pay him spousal maintenance as he's too unwell to work (not entirely true, he has issues with being an arse to bosses to, so doesn't go down well job wise!). I know however that he has been cheating on her for most of their relationship and throughout heir marriage; so my AIBU is to ask if I would BU to tell her about this in hopes that gives her more chance of a fair split and settlement as she'll also have some ammunition? Yes he's my friend, but I feel like I'll be letting down another woman by not giving her the information, wwyd?