Hi Dontknow88,
Do you feel obligated? I don't think that there is a right or wrong here. I think that there is a potential for triangles between you, the friend and family members and in that case I would suggest to not divulge too much personal information about yourself and don't ask too many questions about your ex, stay in the middle by not taking any sides. I think that us difficult to go through a custody dispute, I don't know about you but I wouldn't want additional stress from potential drama added on my plate. I think that the most important thing to focus on is taking of you.
My mutual friends with my took her side and I dud bump into one ex friend that stayed in the middle but that life that we is behind me and I chose to make new friends with my new life. I didn't want to have that connection with my ex, through old mutual friends that really weren't good friends to begin with. I guess a question could be, how much do you value the friendship?
I don't value the friendship at all cause they arnt my friends they are his that he pushed away since he broke up with me (he pushed them away cause they didn't agree with his reasons for breaking up with me, so that started something) so now they are asking me questions cause all they see is a guy going out of his mind doing crazy things.
About his family I think no matter what family will always take family side over the logical side