You didn't say in what position the gent was carrying, but as it got stuck under the chair back, I am envisioning a small of back or behind the hip arrangement with the pants worn way too low, thereby allowing the gun barrel to conform to the "natural curvature" which would cause the top end of the weapon to "ride out" from the body instead of flat when standing.

The belt is important, but the guy needs to get pants with a longer waist, or pull his existing pants up. Having a heavier belt while still wearing the weapon too low on the waist will not really fix the problem.

Dress for carry, not for fashion. That lesson is at least as important as the issue of a proper belt.