Wording carefully what you say to someone will prevent any statement from sounding like sexual harassment.
Personally, I wouldn't say anything to her unless it happens again. She may or not be aware that her thong was showing. Although some of us purposely try to show off to those we are interested in
One of my colleagues told me non-verbally with a hand gesture that my thong was showing. We are friends out of work so something like that wasn't offensive to me. If it had been someone I didn't know the reaction may not have been so calm.