The fact is that one roommate cannot evict another only a landlord has that right. Since she is not on the lease, then she may or may not be considered to be a tenant. However, even if she is not technically a tenant she is, at the very least, a licensee. This is someone who has been allowed into the premises and been permitted to stay. And licensees are given rights under the law.That all having been said, after 9 years, your girlfriend has more than likely established herself as a legal tenant. For example, if your parents allowed her to receive her mail there and use the apartment as her legal address, etc.At this point, your parents should consult directly with a local attorney who can best advise them as to their rights/remedies under specific state law. However, until they do, they should do nothing to distrub her tenancy. So for example, they should not attempt to change the locks, etc. If they do, they can be charged at the very least with unlawful eviction and at worst criminal charges may possibly be brought.