I'm at the pool, and there's a lady here whose swimming costume is totally see through at the back.
I want to tell her because I would want to know if it was me (because this is one of things I worry about at the swimming pool) and she isn't here with anyone other than kids too young to tell her.
But I can't decide if it's better just to leave her in blissful ignorance as I'm sure she'll be quite embarrassed?!
I don't really know how to say it either - "I just wanted to say I think your swimming costume might be on the way out"?