I agree with everybody who has suggested that she get a lawyer. That is step one.

That having been said, it sounds like this guy is going to be a dick about the situation, so your friend might need to "out dick the dick."

I'd double check with the lawyer, but my instinct is to take the boat and hide it. Sail it up to the San Juans or out to Neah Bay. Pay one of the small marinas out there a couple hundred bucks cash to moor it for the winter while her lawyer fights with this asshole.

Then she and her lawyer can say "you want to know where the boat is shithead? Sign this, and we'll tell you." I know coves and bays on islands up there where you could stash a boat for half a decade and nobody would ever find it.

It sounds like this guy is a grade A douchebag and playing hardball is going to be necessary. So, I'd hide it until he agrees to buy out her half or sell the boat and split the profits 50/50.

I don't know the legalities involved here so again, definitely run it by the lawyer, but I can tell you that the waters of Puget Sound and the San Juans still have plenty of places you could stash it to teach this dirtbag a lesson.

Good luck, smooth sailing and following seas.

Agree with Basil - she needs a lawyer, preferably sooner rather than later in case the guy tries to sell the boat himself. I found this guide to WA property law for separating unmarried couples online, and it looks like good basic advice to me (a layperson), but not something that would replace a lawyer. Here is their advice for getting legal help in this situation:

If you have additional questions and you’re low-income and don’t live in King County, call CLEAR at 1-888-201-1014. If you live in King County, you may contact the King County Bar Association’s Neighborhood Legal Clinics (206) 267-7070 to ask for a free half-hour of legal advice. (Ask for the family law clinic.)

As far as payment, it's up to the lawyer but I'm doubtful they would agree to take fees out of the sale of the boat (aka contingent fee basis). I think usually you have to pay a retainer, and it's usually non-refundable. Here is a general description of how lawyers charge. Also make sure she asks what increments they bill in - 6 minutes, 15 minutes, etc.

I wish your friend luck & good health.

She will probably have to get a lawyer. My guess is that the guy will not want to actually sell the boat, so she will have to go to court and have him pay her the amount that the boat could sell for if he sold it right now.

There are other options, including mediation (in-court or out), or even an appointed arbitrator. All of those could be explained by a lawyer.

Not sure of any good ones in that area, but she might want to look into divorce attorneys. They probably have experience with stuff like that (even though it's not technically their area it is related). They would probably be able to give her some idea of her options. If not a divorce attorney, then a simple civil attorney would work.

You noted that they are co-owners. The Report of Sale for boats in Washington state requires all owners sign off before any sale is considered legal.  So, neither of them should be able to own it or even sell it until they can come to mutual agreement.  Of course, this doesn't stop all ignorant buyers and DOL agents from neglecting to check, but it should give her a modicum of legal traction for a lawsuit if he tries to sell it out from under her.

If he wants full ownership and therefore use, he should offer to buy her half out at a fair half price. If he wants to claim full use by passive-aggressively using her as a doormat, she might not feel bad to start the process of selling it out from under him, 

or

she could take soundslikepuget's advice and moor it near the Canadian border to put the whole process nicely on hold.   I know a handful of process server and bounty hunter types that would take on such a gig at just the cost of expenses (train/bus ticket back home from her and a beer from me), considering her story.  Guemes Island is good: no WSF service to it.