I think a plain low absorbancy one would be fine. Be sure her periods are heavy enough...sometimes using them for a too-light period or changing them to often cause inside abrasions and can contribute to toxic shock. They are starting to think that's more of an issue than what they used to tell us (left in too long.) If her periods are really light, she could probably swim anyway. You don't really bleed while in the water (ever taken a bath while having your period? you'll know what I mean)...she'd just have to get in right away after changing and go stright to the bathroom when she got out. Use a dark towel and coverup in case it starts on the way.
They might not be comfortable right away, you might want to use a little ky on them to help her get them in at first.