Stock rebuild? Anything special to it?
In these cases values are usually determined by location. I can find stock rebuilds around here for less than a grand. Rebuildable core engines in the $150-$200 range.
In your case, since you're trying to make a fair offer, find out what's in it for parts (new pistons or not, anything special about the heads, cam, springs, etc.) and then start adding up what it might cost you in parts if you were to rebuild it. I'd offer him up the total for parts, minus about 10% for the fact it's been sitting, but only if you can verify if the initial break in was done right.
That would give you a fair starting price and you can negotiate from there as to what's in your budget while keeping a buddy.