I have not ever noticed any "thong bikinis" on any of the ships I've been on. You will certainly see all shapes, sizes and ages and really you'll see people in bikinis that probably shouldn't be out in public with them...but then again, you'll never see these people again.
I would say that the beaches will be more peppered with skimpier suits (thong, sometimes topless; So to honestly answer your question, I think she'd feel more comfortable at the beach with that kind of suit.
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