They aren’t even divorced yet. I wouldn’t particularly want to meet anyone’s “lady friend”, let alone parent’s, if the person is still married to someone else, particularly your mother.

Saying that if their marriage wasn’t good, it is understandable he is affectionate with a new lady. My stepdaughter loves to visit us from out of state because she likes seeing her dad happy with me as well as seeing us getting out and about enjoying life. She never wanted to visit when her mom and dad were married because they were miserable, dad was mistreated by mom and they did nothing fun because how bad their marriage was. So if your parents are happy apart, it might be better for everyone involved.