I'm looking for an adjective /idiom /expression /phrase with a negative connotation, for describing someone who spends their time mostly with their friends, and prefer their friends to their own family, even though they are married.
The only phrase that comes to mind is a negative one, "He's not a family man."
family man noun [C] uk us
a man who has a wife and children, or who enjoys spending a lot of time with them
It's a very well-known idiom so I think it's suitable to your context.
He/She is not family-oriented.
A family oriented man/woman is devoted to his/her family and prefers to be with them.
- Being family oriented can carry several meanings, depending on the context in which it is being used -- but at its core, being family oriented is appreciating your family, your relationship and the relationship that your partner shares with his or her family. Being family oriented is a state of mind that guides your actions with friends, family and in relationships. Individuals who consider themselves family oriented may come from a strong religious background, may have a tightly knit family, or may long to start a family with the right person. (from cs.com)