Here is a little tidbit from Washton's ATV Safety site.....
http://www.atvsafety.gov/state/washington.htmlAll ATVs must be titled and registered, with owners receiving an Off-Road Vehicle use permit and ORV tag to place on their ATV, unless used solely on agricultural lands owned or leased by the ATV owner or operator.
Your friend may be able to get a temporary permit. Here are the requirements.
http://apps.leg.wa.gov/RCW/default.aspx?cite=46.09.430In the temporary permit statutes, it doesn't state that the vehicle has to be registered. I would say your best bet is to contact the Washington State DMV and ask.
Rand