Thanks everybody! I'll discuss all of your suggestions with my cousin. I know that the most obvious one (taking pills as if she were on birth control) is not feasible, or at least not recommended, owing to something related to the operation she underwent and the condition which led to it. As to the hysterectomy, it sounds like something which might work, though I don't know whether she'd be willing to do so in spite of the fact that she already knows she won't have any more children (something which has already been impossible for the last fifteen years). A couple of years ago we lost a cousin we shared because of a minor operation on a Thursday after which he expected to be back in the office on Monday, and we've often discussed that no surgery can be considered minor. And with her medical history, she might have some extra problem. Taking into account that the reason for the operation would be leisure (diving), and not health, I don't think she'd choose to run the risk (at least, that's what I'd say if I were her).
Incredible as it seems, drysuits hadn't crossed our minds. Maybe because I'm a beginner too (less than her), so much so that I haven't even started buying my own equipment. We've always rented it, and it's always been wetsuits for us. But it's likely to be the easiest solution in the long run.
Meanwhile, we'll have a look at those brands of period swimsuits. If they are really meant to be used in the water, that should be enough.
By the way, we weren't thinking of sharks. We know that's not a problem!
Once again, thanks for being so kind and helpful!