Okay this is something I have drummed into my almost 19 year old daughter for years.
Always wear the correct underwear.
There is most certainly a time and place for g-strings and the like, but there are times when you need decent underwear. For example, when wearing black tights. Have you ever been behind someone at the gym wearing black tights and a g-string? Not pretty.
So that's my advice. Do you have any wisdom to impart?