Yes, sharing an apartment is typical in college. However, when I was in college, it was typical to go to the management and modify the lease to include the extra person. That way they are at least equally legally responsible for everything, and it gives the landlord a reason to care. The landlord may even run a background check, which may have exposed this issue, since you state this was not the first time.

Regardless, tough lesson to learn on not handing out a key to someone without a contract. I mean, that's a level of trust that I don't even have with my friends, much less someone I'm potentially going to room with. No contract, no keys.

I had a feeling there was more to the story. There probably still is, but I'm satisfied now on how this managed to "just happen". I still don't understand how, after the restraining order, this person then claimed legal residence in order to get your friend removed from her legal residence. I also don't understand why this wasn't immediately a phone call to the police to get this person removed from the property.