Dinner w/ the ex

I was speaking w/ my BF today who informed me that his ex-girlfriend would be coming over to his house for dinner. She is close to his mother and sister so they decided to invite her over for dinner to celebrate her graduation. My reaction to this? well I questioned as to why he needed to be present for the dinner. And he said he felt he needed to be there for support because she is not close to her family and has had issues w/ an eating disorder, he felt if he was not there "she would have problems". So I thanked him for telling me even though my first reaction was not good. I am not at all threatened by her but I know he would have some questions if I had a dinner for my ex-boyfriend.
Is it a culture thing? here the parents would ask the son/daughter if it was okay to have their ex over for dinner. Once your an ex, in most situations, the parents respect that and would not invite their daughter/sons ex over for dinner if they said no.
Anyway, I love him all the same.