Most lease agreement documents permit only the signators of the lease to be permanent residents of a leased property.
Other lease agreements may make no mention at all of the number of residents allowed to reside in the space.
Read your signed lease agreement document carefully, as that document most likely contains the answer to such an issue.
If the lease states that only the persons who sign the document may reside there, then your friend’s girlfriend would be required to add her name to the lease if she wishes to live in the house full time, thus creating a three-person split of the rent.
If you are in need of quality documents to create a lease for an apartment, home or commercial, see Standard Legal’s Lease Agreements legal forms software page.