Legal Question in Landlord & Tenant Law in California

There is not actually a law that prohibits charging a larger share to a roommate. The law in S.F. prohibits "unfair" differences in rent shares. So as long as you can make any reasonable case for why the rent was different, she would lose. It is also unclear whether she really can go back and try to recoup excess rent. There are provisions for the rent board to issue a ruling on unfair rent differences between roommates, but a quick (far from exhaustive) check of the ordinance did not turn up a specific right to sue for back excess rent (this being different from a full sub-lease which allows the sub-tenant to sue a master tenant who charges more than the landlord charges the master tenant). Lastly, why do you think she would sue you now when moving out rather than do something about it before now. It sounds like your risks here, while not zero, are pretty low. As far as protecting yourself, though, there really is nothing you can do, except make sure that if any replacement roommate pays more than a direct pro-rata share, that there is a logical reason behind it (largest bedroom is a common reason, but anything that is reasonable and agreed on will work).