Depends on what state you live in, but you can ask her to leave. You have to give her a chance to "reconsider" the lease... i.e. she will quit what she is doing and continue to live there, or break the lease and move out (and forfeit any remaining rent, again, depending on the state). I would say to go to her, tell her that if you smell weed one more time, you will pursue an eviction through the court system. This isn't going to actually happen, but see what she does. If she continues to smoke, give her a 7 day eviction notice, effective upon receipt. Keep up posted.