Tennant wants to have friend move in for longer than 30 days

[Powell DiGangi]

Dear BP

I have a tenant is the only tenant tenant on her lease asking if a friend of hers can move in to the unit temporarily for more than 30 days.  The lease says that she must gain owners permission if the guest is to stay for longer than 30 days.  For a few reasons, we want to deny this request, but wanted to know how binding our denial would be.  What if the tenant ignores our denial and moves the friend in anyway?  Basically, Id like to know how much teeth does our lease have in light of larger oakland CA tenant law.

Any help would be much appreciated

thanks