If it is in pristine condition, and he has already done the 600 mile service... I would say $6500.
If it is in very good condition, but has had the same oil from when he bought it... and it has been sitting with a half empty tank for the past 6 months, I would say $6000.
It is going to cost you a few hundred to get the 600 mile service, and if it was neglected as well as not ridden... you might be looking a few more bones to clean it up properly.