Ask if she is going to pull up her pants or she wants you to park your bicycle back there.
Mm. Just sing “London britches falling down…” 
Glad you liked my stealth brag. 
Just recite this little ditty we used to say as kids:
I see your hiney.
It’s black and shiney.
You better hide it.
Before I bite it.
I would simply tell her that her butt is showing and it might not be considered professional to dress that way. If you have a problem telling her yourself, ask HR to do so. That is one of the HR department’s functions.
I had a female coworker who would wear short dresses and give us crotch shots even with her legs primly tucked together. Yes, she wore underwear, thank heavens. I took her aside and told her one day. I thought she would rather hear it from me than from our boss.
This thread is useless without pictures. 
Your idea of a wet willy and mine are a little different :dubious:
Well.. it could have been stealthier. 