So basically, even though my friend's name is on the lease, he has no right to kick out the previous tenant? The previous tenant is staying there (basically subletting from my friend) without a written agreement.

It seems to me that the previous tenant has no right to be able to stay in the apartment since she has nothing in paper saying that she can stay there.

Perhaps another way to describe it is: does the fact that my friend allowed her to stay mean that my friend looses all rights to the apartment even though the lease is in his name?

Thanks.