I assume she's not wearing glue-on nails. It's possible it's a fungal infection as a
result of moisture. You can try the over the counter: Reclaim AF. It's an
antimicrobial liquid that you can apply after diving and once or twice daily.

Adam

Hello,

I just keep my nails really short when going on a scuba holiday. I find
tugging on wet suits can ruin nails quickly, and hours-long immersion in
water makes nails go very soft and causes them to peel.

One helpful hint is to clip or file down any piece of nail that's
separated from the main part so that it doesn't engender a greater rip
along the nail.

Also, hand lotion that also says "nail strengthening" is good to use at
 night, since the irony of wet nails all day is that they actually lose
water and get brittle.

redcat