"john smith" wrote

Certified? No.
Possible? Yes, but you will finish much lower than where you started.

Don't encourage that type of behavior with a yes. If he had to ask, he is
probably not a competent aerobatic pilot, and a blown barrel roll could
stall the plane, or overstress it, so it fails then, or later.

I'll bet he is not going to wear a parachute, either, which is required.

The answer to rolling a normal category plane should be no, always.

We don't want his, or someone else's blood on our hands.
--
Jim in NC