As a general rule, mixing antidepressants with rec drugs like
amphetamines is not a good idea, but without knowing what drug your friend is on I couln't tell you any specific dangers. There are many different classes of antidepressents (MAOI, SSRI, Tricyclic etc.) as well as many drugs perscribed as antidepressants that are chemiclly unrelated to all others. They all have different modes of action, and regulate different chemicals (or combos of chemicals) in the brain, so more info is necessary to know just how dangerous the combo might be. Mixing amphetamines with MAOI's is a potentially lethal combo. You are right to be concerned about your friend's health. Try and find out what drug she is perscribed so you can confont her with specific dangers from an informed background, and they'll be much more likely to listen.
Information = Freedom