There was a recall in reference to the high pressure fuel pump on certain F40's. If you had a PID number of the outboard, I could run it and see if it's eligible. If it is, it's leverage that you can use to negotiate a better price. Say, offer the owner a few hundred less to cover the cost of the fuel pump and the aggravation, then take it to the dealer, have them perform the recall and away you go.
That said, if he's not used it in 2 years, the gas is JUNK. It's entirely possible to plug all 3 injectors and destroy the fuel pump and/or it's inlet screen/filter. Happens often. 10 days and the fuel quality is degrading, which is why I recommend folks if you're not going to use the fuel up in a week or so, dump it into the lawn mower or something that will (truck?) use it and start fresh next time you're heading to the water.
I have a pint pickle jar at work that I use for taking fuel samples. I tested fuel from a F25 earlier today and filled the jar about half full (or half empty however you want to think of it). In 4 hours, 50% of it (or more) was gone due to evaporation. And what was left smelled really weird. That is what happens to fuel inside of a carburetor, or anything that is vented. It evaporates and leaves behind the chemicals that will not evaporate; and that's what plugs stuff up. Kind of like salt water. When it evaporates, it leaves the salt behind. Same principle.