Does anyone have suggestions on who they would handle this?

Couple weeks ago, I picked up SD from school as a favor to SO. On the way home, she tells me about an evening after school trip happening on one of SO's weeks. BM had already signed the permission form and told SD that she could go! Uh, hello dumb bitch. You don't get to say what she can or can't do when it's not your week. She also didn't offer to pay for the trip. Great...So I told SD that since she was with her dad that week, it's not her mom's place to give her permission and that she would need to talk to her dad about going.

(Side note here that I kind of felt I was putting SD12 in the middle as well, but I feel that she is old enough to know that one parent doesn't get a voice in the other parents' house. Maybe I'm wrong...)

The trip lasts until 9pm at night and SO lives 30 mins away, it's doable, but what precedent is it setting to allow her BM to grant her things? Last I heard, BM told SD that she would pick her up from the trip and she could stay with her that night, didn't talk to SO about it AT ALL!

I get so pissed off because it makes SO the bad guy! "oh well since you're dad is mean, I'll make sure you get to go."

Suggestions? Should SO have a talk with SD about activities on his weeks? I mean, he can't force BM to co-parent with him, but SD is old enough to know the difference. What he should say to BM?

My reation would be to text, not call, BM and tell her that she is not allowed to sign permission slips for SD on the weeks that he's got her. I would also have a talk with SD telling her, that while he will allow it this one time, but if BM agrees and signs for something on one of his weeks again, she won't be able to go. Her afterschool thing doesn't often have late night events, but BM does have a habit of trying to tell SO what happens in his house/time.