Me, I would use this to my advantage to maybe reduce the price of the car a little You know it runs but of course not like it should and it's going to take (a little) work or $$$ to get it corrected. Doesn't seem to me like it going to really be a big issue to resolve. A new carburetor, a carb rebuild, a carb swap with a known working used one. Ask the owner if with this issue they would consider less for it? What happens if you get it running correctly and the guy then figures it's worth more than what he was hoping to get, then you just fixed it for him and then wants more $$$. I'd see if he will take less and get it home and get it running correctly. (and don't offer any help at this point on what might be wrong, again

Sorry if I'm misreading the situation here but is seems you are fixing something that you don't own at the moment?

Mike