Hi my girlfriend is on metronidazole 400mg two pills a day can she smoke marijuana while being on these antibiotics?

Not adviseable: She can and maybe will, because she's in charge of doing or not doing this. However, smoking marijuana is not advisable due to impact on mental, emotional, and physical health. In some, it can cause psychosis. Marijuana can also impact immune response. Since she's presumably being treated for an infection, this could be pertinent.