I sympathise biglips. My dsd had horrendous nits - we would comb out literally hundreds - and her mum refused to do anything about it. She comes to us every 3rd weekend and for half of the holidays. We could treat and clear her of them if she was with us for the week, but she would go home (where she had 3 siblings) and, of course pick them up again and gradually over the next few weeks we would see them getting worse and worse.
It was so frustrating, we never really wanted to raise it with dsd's mum as we try to avoid anything that will cause friction. But it really wasn't fair on dsd to be so badly infested. She would say things like she didn't want her hair tied up for school as the other kids saw the eggs and teased her .
If dh tried to talk to his x about it he would either get a mouthful, the door slammed in his face or she would refuse to answer the phone. I would spend hours in the shower with dsd combing them out, and of course once we had dd she started getting them. DH gets off scot free as he's completely bald.
I've no idea what changed for dsd's mum, but she suddenly seemed to start making an effort. Dh was able to talk to her about it, we sent home a nitty gritty comb and dsd started going to a school where they had to have their hair tied up. Maybe that school was stricter about children with nits, I really don't know. I never thought I'd see the day when dsd would visit without the nits, but I have! She had them again recently and ironically her mum told dh that she'd got them from a friend, and was really indignant that the friend always had them therefore dsd kept getting them!!
So, lots of sympathy from here. Have you tried talking to her mum about it?