Ex keeps asking me to step in

[localhere]

I know it’s an easy answer ‘just say no’ but that doesn’t take into account the guilt I feel for not stepping in.
When we split two years ago ex insisted on 50/50, consequently kids spend a week there and a week here. Recently DD12 has started ‘being ill’ on her weeks with Dad but instead of him sucking it up, being the baddie and either sending her to school or taking time off with her, he’ll text me with ‘DD is saying she’s not well but I’ve got stuff on at work’ and expect me to pick her up (I WFH) and care for her for the day. I know if I say no he’ll tell her that I ‘refused’ to have her so she’ll have to go to school. There have been a couple of times in the past where he’s just gone into work anyway and left her by herself all day and we had words about that not being acceptable. So now he expects me to deal with it when it happens.
If I responded with ‘She’s just going to have to go to school, give her some meds and take her’ he will turn it into ‘So sorry darling but [mean old] mum says you have to go to school’! For what it’s worth she rarely misses a day of school on her weeks with me, she has some great friends and is doing well at school. Why does he not want the responsibility?

I mean, he is to blame. He’s not a bag of lettuce! he’s supposed to be the other 50 of the 50/50

Respond with something like "work has sent me on a course in x location (at least 4 hours from home) , " l'm at a funeral" and "tough, man up and actually parent your child!" (Most men hate their masculinity being called into question).

She IS having stomach pains, however this is one thing on a list of things, I suspect are mental health related as opposed to physical.
Just a heads-up as not all GPs are aware of this. Migraines can manifest as stomach pains in tween girls. I am wondering if she is getting hormonal migraines as her body hasn't adjusted to puberty yet. Girls do outgrow them around 14 to 15 although some do end up with monthly migraines.