How to react to Ex &DD

[MissKST]

I know this technically may not be the correct place to post, (maybe lone parents? But I'm new so I don't know)
Anyway my DD goes to her Dads on night a week, some weeks he'll have her, some he's 'busy' Our DD has complex needs and as a result of an illness she has delayed milestones, however she's three, in the past 6months I've gotten rid of her dummy (she only had it to sleep or if she was ill) and toilet trained her for days and nighttime. It's took a while by we did it. Yesterday she came home from her dads with a nappy on, (5.30pm) I asked why and he says he 'doesn't want her to risk having an accident in his mums house or in his car' I said this will be so confusing to her! He said 'it's okay I've told her she's not allowed to do anything (in the nappy) and to tell me when she needs the toilet! I'm literally losing the will to live trying to drum in into him! He refuses to let her be nappy free in his time. AIBU to say he can't see her unless he's going to let her be nappy free?

Just to add, I'm not the type of mum to say 'right that's it you're not seeing her' but I feel it's wrong and he's going to confuse her so much!

Ate you on good enough terms to explain how it will set her back/confuse the issue.

I've explained so many times, I've found him information on google and sent that, I've tried speaking through it and saying why it would confuse her. I've even spoken to his mum and said surely you know he shouldn't be doing it? And she just says she can see why he's doing it

I had this issue many years ago when at the same stage with my DD. I'm afraid that I did tell him unless he was willing to stick to the things I was doing to toilet train her, as in not be lazy and just bung a nappy on, then he'd have to see her when I was present. Myself, my mum and nursery were all approaching it the same way but he got 'embarrassed' when she had an accident at his brothers house. So he bought and put on a nappy and said it was his right. So I said unless he was going to get involved, I'd be present and I'd deal with any 'embarrassment', until she was a bit older. I followed it up by saying if he insisted then he could be the one to explain to school, HV and other services why she was still in nappies when she should have been toilet trained.
I'd stand firm OP, if she's managing and it's a good time for her, then insist this happens.