1. If the buyer is present in WA with the intent of making WA a home, the buyer is a resident of WA for the purposes of firearms transactions. That is 27 CFR 478.11.
http://ecfr.gpoaccess.gov/cgi/t/text....2.1.1&idno=27
2. There is no "verified or recorded" transfer in WA for private transactions. The two parties can file a Pistol Transfer Form with the state, if they choose too. Why they would choose to is beyond me. The state maintains a registration database of handguns purchased from FFLs, but that registration is NOT required for private transactions.
Here is the form:
http://www.dol.wa.gov/forms/652004.pdf
Notice the bold and highlighted word "VOLUNTARY" on the form itself. It is not required and certainly not recommended.
3. There is no 90 day residency requirement to purchase a handgun from an FFL or from a private party in Washington.
4. 18 to 20 year olds are NOT prohibited from purchasing firearms, including handguns, from private parties in WA. They are prohibited from carrying those handguns in public for self defense.
5. Also, if your friend happens to be a military member, he also is deemed a WA state resident if he has permanent duty station orders to Washington State.