You are correct, sir. Lawyer here. Adverse possession must be "hostile," meaning contrary to the owner's right or title to the property. An invitee (such as the ex-boyfriend) could not qualify. Adverse possession also must be continuous over a far longer period (20+ years in many states) than what has occurred here. This is the general, common law understanding of adverse possession in the U.S.; it's possible California has modified it. It's also possible that a resourceful lawyer could come up with some other theory to cause delay and expense for Unicorn; adverse possession won't fly, though.
Sorry this happened, @Unicorn, and wish you the best in resolving and recovering from it.