Legal Question in Real Estate Law in California

You aren't obligated to report her, but neither should you actively assist her. Unless use of your garage is part of the rental deal, you might want to advise her not to park any car in it, whether subject to repo or not.

Also, your roomer is probably not legally a tenant, but rather, a lodger. As such, she has fewer rights than a true tenant. Among other things, lodgers are not entitled to formal eviction proceedings with long prior notice and rights to an unlawful detainer proceeding.