What would 你 do if 你 saw that a friend had a piece of 食物 stuck between his 或者 her teeth?

If I could do so without embarrasing them, I would tell them.

Tell them "Yo, *Friend's name* 你 got a little somethin in your teeth. :3"

Hehe.

I would tell them straightforward that they have something stuck in their teeth and point it out.