Wankerjake, I'll try to help you with an educated " guesstimate " on the ballpark value of your friend's Vaquero, based on your replies ( if you know ) :
Was he the original owner ?
Is it new & unfired by him ?
If used / fired, what is the overall condition of metal and wood finish ?
Condition of bore, rifling, crown & chambers ?
Lock up fairly tight with good timing ?
Screw slot condition ?
Any known problems, inside or out ?
Any idea of amount of use it has seen ?
All original parts ?
Stainless or color case hardened ?
What is the barrel length ?
Obviously, condition will play a major role . Type of finish and bbl length will affect the price as well. Stainless pieces bring more and usually, the shorter 5 1/2 " & especially, the 4 5/8 " bbls bring more than the 7 1/2 " models.