[This might be a bit long and detailed, sorry, but I don't want to drip feed.]
My Mum and Dad seperated when I was about two and a half (over twenty years ago now). From what I've been told by both parents it seems that my Dad was largely absent, due partly to his work as a Police Officer but also partly to the fact that he liked to play rugby and go to the pub, and my Mum was a control freak and very difficult to live with. My Dad met someone else a few months later and will have been remarried nineteen years in August. My Mum has never remarried but has had an on/off partner (they spend a lot of time together, go on dates, but don't live together or want to) for a long time. My brother and I have always lived with my Mum, and seen Dad regularly-ish.
When I was growing up I had a difficult relationship with my Mum. I don't want to go into too many details but there was some abuse (physical violence and a lot of emotional bullying) and she kicked me out when I was eighteen. I slept in my car for a few months, got heavily into drink and drugs and went a little off the rails. Since DS1 was born, however, Mum and I have patched things up pretty well - we still disagree sometimes, but only on a normal level IYSWIM? She and I actually get on quite well now, and she's a brilliant Grandma. She and my Dad still don't get on but she chooses to just not see or speak to him unless they happen to be invited to the same family get-together (this doesn't happen often - DH and my wedding, my brother's 21st, DS1's birthday parties, his Christening.. that's about it really). If they do see each other she will be polite and friendly, you can't fault her really.
My Dad, OTOH, is a bit less polite about it all. I get that she was horrible to him, and that's why he left, but he's been away from it for twenty years. I put up with it all for most of my childhood and I've still been able to be grown up enough to forgive her and move on. He doesn't seem able to do the same thing. He makes snide remarks about her, still, to me. Just little things but it's rude and I don't like hearing those things said about my Mum. He also makes a point of leaving early if they happen to be at a family get-together at the same time - he left DS1's 2nd birthday party after forty minutes because he 'finds it difficult to breath the same air' as my Mum. I know there must be a lot of bitter feelings between the two of them, but after twenty years I feel that he should be able to go without making sarcastic and rude comments about her. Or at least making them to someone else so I don't have to listen.
AIBU, or is he?