That's gross.
Does she know you know? When you catch her doing it, tell her : What the hell are you doing?
There must be an explanation. Cause that's really gross.
I'm not sure if this can cause acne but it is not the healthiest thing to eat.
Zits are dead skin, caused by sebum in your skin and in the zit contains several bacterias...
Tell her that... Tell her that she's eating dead skin glued with sebum and it contains a bacteria known as Propionibacterium acnes...