She should be fine if she goes in and registers it now. The DMV will want to verify her new address, but they generally won’t try to verify how long she’s been living there. Worst case, they’ll charge her back registration fees.
However if she gets pulled over and the cop susses out that she’s been living or working in California for more than 20 days, I think it’s something like a $400 fine. In most states she would almost certainly get a warning or a “fix-it ticket” where the fine would be dismissed as soon as you did register it, but unfortunately there’s a long history of people driving with “license plates of convenience” to avoid fees and emissions regulations in California so they’re usually a little more hard-nosed.
Realistically, though, so long as there’s nothing glaringly obvious like a CA driver’s license or phone number and she doesn’t mind… shall we say… telling a few little white lies of omission, the chances of getting caught are pretty darn low. If she does decide to become compliant though, she should remember to change her driver’s license too, since that’s another big fine.
I think she already got pulled over because she said the fine for not registering soon would be about $900. I’ll have to find out if she got anything that might already be in the DMV’s system. I hope that’s the worst case scenario!!
I meant to preface that my dealings with California’s motor vehicle laws are about a decade old, so things may have changed a bit. Although maybe the $900 is the penalty for both the driver’s license and the car registration?
At any rate, if she got a fix-it ticket, then yeah she needs to get it taken care of to avoid the fine. The DMV won’t pile on any additional fines, except maybe making her pay the back registration and they probably won’t do that unless she volunteers the information. If it was just a warning, she can probably continue to take her chances.
Of course, bringing in a car into California from out-of-state can be a hassle and fairly expensive so she might want to figure out what all that will entail. If I were going to be leaving at the end of the year, I’d probably just take my chances.