Hm. A bit of an akward situation. Could really use some neutral advice from someone else

We went to weekly appointment with S. Usually all fun and laughs, but today she was somewhat upset and not cheerful at all.

She said she was tired, but shortly after made a comment about how she didn't know what to do this weekend as S was going to stay at my place and she didn't have any plans.

I stayed positiv and cheerful, but didn't offer to come to the rescue. I'm not the-last-resort-guy.

But, The same day as our meeting she is putting final touches on one of her tattoos. The guy she was flirting with while we were together works there.

So today before we go our own ways, she hesitates and ask if I can pick up S from child care and have him while she gets tattooed. I said I could have him the whole day, but she had the day off so that was out of the question.

So to sum up :

-Obviously it's purely so she can do her own thing.
-I can only have him while she gets tattooed.
-The "OM" is working at the same place, not sure if he is working that day.
-But, I'm also depended on her being flexible with me sometimes

My take : I will say no. Not out of jealousy or spite. But because I have to have boundaries. This seems to me like a true classic chit test! I could not respect myself if I said yes. Either I have him most of the day or not at all.

I'm sure she will get angry, upset and feel like I'm just ruining stuff for her, but so be it.

I'm thinking of saying something like "I've considered and come to the conclusion that I can have him most of the day or not at all. I see it's purely for your own sake, and on top of that you are going to the workplace of the guy you were flirting with while we were together while I babysit our S. I'm not saying no out of jealousy or spite, but out of respect for myself."

What do you guys think?

Together for 8,5 years.
S2
Interest in OM.
She left 29.09.12 b/c we couldn't work things out.
No signs of OM, not digging.
Living in seperate homes, sharing custody.