My first suspicion is an intake leak. The 91 has had plenty of time to dry out & crack a few intake gaskets. Try tightening the intake manifold bolts (BE VERY CAREFUL ABOUT NOT OVER TIGHTENING, ALUMINUM STRIPS EASILY) to the maximum limit of the specifications in the Haynes manual. Note that these specs may be in inch-pounds, not foot pounds. Overtightening is BAD. If that doesn't work, maybe a set of gaskets?
He might be able to find a leak with a can of carb cleaner. Spray it in various seams of the manifold & intake assembly to see if the idle gets worse or the engine dies. Check any hoses & their associated clamps that are connected to the intake.
Good low dollar place to start.
