roommate won't move out, what should they do?

hello, i'm asking this question for my friends. they are in california.

my friends (a couple) rent a 2 bedroom apartment and sublet one of the rooms to another person. before this person moved in, they informed her that there will be guest staying for a while (~ 1 month) in the living room and she agreed. (the rent they charge is pretty low and she probably thinks it's hard to find a better deal later).

however, soon after the guest came, she decided to move out (maybe thinking it's too crowded). that is at the beginning of a month and she did not pay the rent for that month (she had delayed payment before). she stayed at her friend's place and come back occasionally to pick up stuff. now, one month has passed and she still hasn't paid the rent and argues that she won't move out for another month since her new place won't let her move in until then.

my questions for my friends are:
1. is it ok if they do not give her back the deposit (=one month rent) to compensate the rent she already missed.
2. if she still doesn't move out one month from now as she promised (she had promised to move out earlier and didn't do so), is it ok to throw her stuff away. (last resort of course).

the tough part is that everything was private agreement and there was no written statement. moreover, my friends' lease with their landlord states that there should be no sublease. so they are afraid of being expelled from the apartment if the roommate tells.

any suggestions or comments are highly appreciated!