Serpentine Belt Riding Off Pulley

The son of a friend has a 92 Olds with the 3300 engine. Recently the
serpentine belt jumped off. He replaced the belt and the tensioner and the
belt now rides over the back (nearest the engine) flange of the harmonic
balancer pulley. He reinstalled the original tensioner and the belt rides
over the back flange of the harmonic balancer pulley. I have reviewed the
belt routing with his dad and the belt is routed correctly. We can't see an
obvious problem and would appreciate any help with this.

Thanks.