To expect DH's ex to tell us when DSD has head lice?

Children are not excluded from school for headlice.

Sofia, can't remember where you live, but in the UK headlice are absolutely not a reason for staying off school. And there are other options as well as combing - Hedrin, for instance. Agree it's mean of ex-wife not to give warning, though.

You are so totally not being unreasonable. I have been there!!!! My dh's ex sent his kids every weekend for close to a year with lice. Every weekend, I would have to comb their hair, my hair and my ds' hair. Dh has skinhead so not an issue for him. I caught them and ds (18mo at the time) caught them. I was pregnant and couldn't even use the shampoo, but had to try to comb my own long wavy and uncombable hair. It was awful. And it was very very clear that dh's ex was doing it on purpose. I finally put my foot down (after almost a year) and said that dh was going to go up there (2 hour drive) to pick up the kids and do a head check. If they had the lice when he arrived, he was going to turn around and come back without them. He only had to do that once, before his ex realised that we were serious and got rid of the lice once and for all. Getting rid of lice can really only be done if you systematically and regularly comb all the members of the family for hours until the eggs and lice are all gone. Dh's ex never wanted to be bothered doing this. (didn't help that she had 4 other children in the house besides dh's 2, so it was a lot of work.)
A child is not supposed to go to school if they have lice because other kids can catch them. She certainly shoudln't be sent to you without at least a warning and you being give the choice of whether you wish to either treat the lice or just put up with them.
By the way, it certainly helps to make sure that your hair is always completely tied up when you are around your sd and that her hair is always completely tied up too.
And make sure you check yourself regularly for weeks and weeks after you are sure that hers are all gone. Much easier to catch them when you only have a few than when you are riddled.

YABU. If you are concerned check her hair yourself or get her dad to check. Headlice are notoriously difficult to get rid of and you are being very unreasonable to suggest that sd doesn't visit unless she is clear of them. Help her to get rid of them. Tie your hair up to avoid catching them.

How did you check? Rang up and asked? And she said "yes I knew she had them but decided not to mention it"?

Very discourteous if so

I try to keep on top of DD, but then suddenly you realise they are back and you didn't notice. Its probably not intentional.

Yes you probably are. Unless she knows that her DD has headlice and is deliberately not telling you, chances are she hasn't noticed.

All depends on how old your SD is as well - if she washes her own hair, then it's even less likely she'll notice.

I wouldn't assume she'll actively want her DD to walk around with lice - if my dc's have them, I wet comb every day and would want anyone caring for them to do the same, just to ensure new eggs aren't laid.

no she should tell you but I suspect many exP would love to infect their exp new dh/w