He might own it, but that doesn't mean he has a right to move back in. If he is not a resident, he can be arrested for trespassing or unlawful entry if he tries to make entry. Though, as an owner, he may have some rights in WA to make entry with proper notice, etc.
Your neighbor might want to speak with an attorney ASAP to determine what her legal options are in regards to keeping him out of the home.

