I would ask the doctor or at least a nurse. I know for sure that antibiotics make pill almost ineffective, but I don't know about the one your gf is taking. I would just ask the doc to eb sure.
I am not aware of any interactions between remeron and contraceptive pills. ALWAYS check with the prescriber before mixing any pills. This includes dietary supplements as well as pharmaceuticals. Antibiotics and vitamin E are the two that most often get us in trouble.