748S is a reliable bike. However, the 748 has some things you should be aware of.
As with most Ducati motorcycles, they have tedious maintenance schedules. It's a superbike, so belts every 2 years, valve adjustments every 6,000 miles. Keep on top of the fluids, and the clutch reservoir.
The biggest thing with the 748 was the rocker arms. They had a major recall with the rocker arms that would make them flake, and essentially cause damage inside the motor. Some people got lucky and had no problems. Others were not so lucky.
If the owner was smart enough to have this checked, they should be replaced. Make sure if you are looking at the 748 that it is well inspected and that the service records are clear, and state that it had the recall fixed, otherwise you'll be out of pocket and have issues down the road.
On top of this, this particular generation of Ducat is quite prone to electrical corrosion, so make sure to check around the areas of the frame, the battery for any corrosion, as well as the relays and fusebox to make sure it's clean. Even if the bike "LOOKS exceptional" the underlying is sometimes deceiving.
If you are unsure about what to look for generically in a bike, have the bike taken to a dealer, inspected (worth paying for) and then consider it.
Either way ensure the recall stuff is taken care of FIRST. Then check the other stuff, wiring, service records, any engine noise, etc.
They are wonderful bikes, quite reliable, you just need to keep on top of them. Otherwise they are quite sound and fun bikes to ride. I realize it is your friend's bike, but unless he's a Ducati mechanic, and you've seen him do all the work himself, make sure you still have it looked into very thoroughly. The Ducati is a great manufacturer, love mine to death. But buying a bad one with bad history can be the difference between a pleasant experience, and a money pit from hell.