You piggybacked your question on to a post that was 5 years old. I moved it to its own thread so that everyone would be able to see it.
Even though the girlfriend does not have a lease and does not pay rent, she is a resident and must be treated as a tenant. The owner has to give her a written notice that her tenancy is terminated and she must move out in x number of days. The number depends on the state law and is usually 30 or 60 days. Tell us the state and we can tell you the number of days.
If she doesn't move out by the end of the time period then the owner must file a lawsuit for eviction.