I have a friend that says a Commercial Jet Aircraft can be flown upside down for a sustained length of time if necessary. I totally disagree with that opinion and have a bottle of "Patron" riding on the outcome! Who's right on this one?

Definitely maybe: Here is a discussion Inverted Flight — Tech Ops Forum | Airliners.net

Personally I suspect that if the fuel and lubricants still flowed, and you had a smooth pilot, most of the moderate-sized ones, at least, could be flown inverted.

He didn't tell you he just watched episode #1 of Human Target, did he? FOX on Demand - Human Target

Wonder if the show asked Boeing.

Sustained is the issue I look at, one of the things I learned in ground school many years ago, was the principle of lift, what causes the plane to stay in the air. Which is caused by the flow of air around the wing, and its specific shape, you would not get the same lift if the wing is upside down,
Assuming you get "some" lift with a upside down wing, there would be increased power needs.

There's also the matter of 139 passengers being upside down for a sustained amount of time.

FR_Chuck is absolutely correct! There is NO way possible for the plane to remain inverted in flight. If it were at 36,000 ft and flipped over, technically it could stay inverted until ground contact...