General

This friend I know

This girl I am pretty good friends with (I don't visit her at home or anything nor am I ever going to date her)
She is really nice, however, the problem with her is that her shirt is always falling low...at her chest. Like her breast and sometimes her bra is exposed.
Nobody says anything because nobody wants to be mean to her (she is really nice) by saying that.
Now I am a guy and we have 3 classes together. However, how do I tell this by not looking like a perv?
I hate it when I see other guys looking at her.
I really want to tell her mainly because other guys always get near asking her pointless questions and looking down her shirt
(She isn't dumb or a s***, she is just REALLY blunt)
She thinks she is ugly when she really isn't.
Now time for the question, but how do I tell to pull her shirt up. Btw she has really large breasts.

PS: I know you are thinking I look down her shirt like a perv, but really I don't. Frankly I am not a big fan of breasts so yeah. Also I know I should have put this on basil but I don't know any sites.

Any help is appericated