Gosh that is a tough one
I don't have an au pair but as I understand it there is an element of pastoral care around the relationship between au pair and host so i would have thought that you probably should give her some advice.
I am not saying that you should go out shopping for, ahem, items together,but I would have thought that you could point her in the right direction.
Can I ask how old she is?
If she was 19 or older I would be quite relaxed, if she was 16 I would be less so.
T
Ps I suppose it could be worse, she could ask your husband!