Legal Question in Landlord & Tenant Law in California

If both of them were on the lease, they are both jointly and severally liable to the landlord for damages. Being primary or secondary has no relevance, and in fact I have never seen a residential lease that designates a primary and secondary lessee. They just list all the parties to the lease, and then any other occupants who will not be parties to the lease (usually only minor children). So your boyfriend is either a party to the lease or he is not (if for some reason the landlord let him only be listed as a resident but not a tenant (I've never heard of such a thing)). So either they are both jointly and severally liable under the lease, meaning he can sue one or the other or both, or she is the only one actually on the lease, meaning he can only sue her under the lease. There is no way to tell from your question which is the case. In either case, however, if she gets sued for damage he did, she can sue him for indemnity. It sounds like that is what her lawyer is preparing to do. It makes no difference who is "primary." The ultimate issue is who did the damage. The landlord can sue her even if your boyfriend did the damage, because the landlord can sue anyone on the lease for damage to the unit. But that doesn't mean she can't turn around and sue your boyfriend for making her liable to the landlord for the damage your boyfriend did. That is what indemnity is all about: a person who is liable under the law for something they did not actually do themselves has a right to sue the person who actually did the thing that created the liability.