The best you can do is try to persuade her to discuss this with a therapist or physician. Many people on medication get pregnant, and they are often able to stay on the medication after discussing the risks (not as great as you may fear) with their doctors. Plenty of pregnant women continue to take anti-depressants, for example.
If she were using illegal drugs, you could report her (though I'd counsel against it) to Child Protective Services authorities. However, if she's on prescription medication, it's really not your business to interfere, beyond trying to convince her to discuss this with medical and therapeutic professionals. After all, they will see that she is pregnant sooner or later.
So will the father. She needs to tell him, so urge her to do so.
If you really think she might hurt the baby (by, say, attempting an illegal abortion or something similar), you could conceivably contact her therapist with the understanding that the therapist could not actually discuss her case with you. All you could do is provide the therapist with the information.