Rape her, then kill her family 
Or...
Ultimately this stuff belongs to your girlfriend so if this 'friend' leaves with it surely that must constitute theft. Whether the police would involve themselves in this matter, I don't know. Your best bet would be to go around to this girl's home and demand she return everything. Do not leave until you get the lot.

Or...
Ultimately this stuff belongs to your girlfriend so if this 'friend' leaves with it surely that must constitute theft. Whether the police would involve themselves in this matter, I don't know. Your best bet would be to go around to this girl's home and demand she return everything. Do not leave until you get the lot.