friend renting to friend

I am a college student and need to write a paper. I need to research a law that pertains to a situation but don't know where to start as far as a law or regulation. Here is the situation I wish to research (what law should I look up?
" A friend is a tenant, on the lease, utilities in her name, phone, everything. She is the sole responsible party to the landlord. Another friend pops up one day and needs a place to stay for a short period in order to get back on her feet. An oral agreement is reached between the two friends about splitting rent and that the arrangement is temporary. A month or two goes by and the friend begins failing to keep up with rent and no longer has income but refuses to leave the property. The landlord has never been contacted about the short term stay as the tenant didn't feel it to be nesesary based on the short term agreement. Eventualy police are called in order to remove the newly moved in friend and the actual tenant is told that as long as the newly moved in friend has even a sock in the property she can not be removed unless the tenant takes her to civil court to have her removed." What laws could I research to write a paper based on this situation?