thats a tough one.
To get the best price on a MC it has to be running and licensed so buyers can take it for a ride, make sure its all solid, have a good maintenance record... that is the best situation for a seller.
The price goes down from there: if the bike was left somewhere with no records or title (estate sale or abandoned)
if the bike is not running (dead battery...)
if the bike is known to have a problem
and worse case, it has been tinkered with, partly taken apart... basket case.
You are close to the high end. If the owner took it to a shop and a mechanic could say for sure whats wrong, then he could put a solid price on it, have it fixed at his expense, and then sell it for top dollar. The cost of fixing it gets passed on to the buyer (you).
If you would rather fix it yourself, then its a matter of opinion of what its worth, and there is some risk, that he might sell it too cheap, or you might pay too much.
The best thing I can think of, since you know each other, look the bike up on one of the used bike price websites (or on a few), agree to what the bike would be worth if it was in good running condition, and then if you think you can fix it yourself, you do the work, split the difference on the cost (parts and your time), so you save some money by not paying a mechanic, and in effect he pays you half of what it would cost him to pay a mechanic.
I think with only 185 miles on the odo the owner is looking at it like a brand new bike, but price is primarily based on its age, and then condition.
Its possible you two may not be able to agree on a price that will make you both happy, because if he is willing to let it sit in a garage unridden, then why sell it?
I dont know about where you live, but here in upstate NY a LOT of people buy new bikes, ride them a few hundred miles a year, and after 7 to 10 years they still have the original tires and less than 10,000 miles, and you can get them for 1/3 to 1/2 the price of a new bike. And they still have 90,000 miles of useful life on them.