My Mum left my Dad when I was 12 and moved to another city with her new partner. They married but divorced after 15 years or so and she is now remarried. I always got on well with the new partner and he was like a father figure to me in some ways. Now as an adult, we keep in touch and meet occasionally (like once per year). My Mum thinks I’m totally unreasonable to do this and I shouldn’t be seeing her ex’s.
YABU - You shouldn’t have a relationship with your Mum’s ex
YANBU - It’s ok to have a relationship with someone in your life for many years.
In my opinion, he is a good man. She left him if it makes any difference!