Being a divorcee myself, I definitely agree with you. If Sean had done terrible things (e.g., been violent against her; kidnapped the kids) then I would see it differently...
I lost most of my friends in the early stages of my first marriage because my husband was abusive. The people I was allowed to continue to be in contact with were all his friends and they stayed with him after the divorce - most are not in touch with me at all, none can I call friends today. One of these was my close friend for nearly 15 years, who I called daily when she was suicidal and afraid to talk to anyone else about it (including her husband) I even convinced her to get treatment... (She's gone on to completely get her life back together, including having 2 more children.)
Your friend must be worried about you choosing sides. I would just assure her that you're not a part of their divorce and that you can be a good friend both to him and to her...
Good luck!
Rachel