First, the flow doesn't STOP just because your swimming. Well, that's what I was told a couple of years ago by my considerable older gyn. She said it may not be a heavy gush that is noticable, but it IS ABSOLUTELY there and at the very least trickling out! Thinking about this issue may have changed since then, but that said, it is a public safety issue for everyone, not to mention very gross. Do you want to swim in water with someone who isn't using flow protection? No offense intended.
Give her the choice (after discussion with a gyn, of course) to use flow protection (tampon, diva cup or something similar) or have her sit out.
edit: Somehow I missed your 'so what happened'. Sorry! You know, it may be easier for her smaller frame to use the OB type tampons where she's not trying to manuver cardboard or plastic. Plus, they tend to be shorter but wider so it may not be so uncomfortable if she has a shorter 'canal' (for lack of a better or more educated word, again, sorry, I drew a blank!).