Hello and welcome.
It truly could be any number of things (including what you already mentioned. Maybe even as simple as a vacuum leak
Without spending some time with it, we'd all just be throwing ideas at a wall and hoping one "sticks".
When buying used bikes (of the F4i's era), I go by the Thirty Five Hundred Rule
If you buy it for $2500, expect to put at least
$1000 into it... and don't expect a perfect bike for $4K either.
If he's your friend, maybe he'll allow you to do some troubleshooting BEFORE you lay out the cash. If not, re-evaluate your friendship, look for another machine, or just roll up your sleeves and (if you're up for a project and can get it "cheap" enough) with the help from Fireblades.org, a Sevice Manual, patience, and some coin, get after it
Just a humble opinion from a like-minded motorcycle enthusiast