Since that person is not on the lease, she has no rights to the property other than collecting her belongings and moving out. Your friend can pick up the phone and call the police to have her escorted out with her belongings. You and a friend should be present while she is collecting her belongings to ensure she doesn't steal anything, but most importantly for you to have a witness, to vouch for an amicable move.
Since the girlfriend is now unwanted she is considered a trespasser and can be escorted out by the police. If she were paying part of the lease, she could argue she had some rights to the property, however you can argue these where gifts as she is not included in the lease and she was under no obligation to pay under the lease as your friend is.
You will need to contact the police to escort her out. You can try one more attempt to have her move out on her own, write a letter and request her to move...(with this you have other evidence that you made an amicable attempt).
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