Are you sure, "reed valve"
I thought a reed valve is something that is used in two stroke engines.
Is the clicking perhaps a valve lifter? When they get old, they don't work as well, and they are supposed to maintain close contact with the valves and camshaft lifters. When they wera out, they allow some space between the valves and lefters, this could be the clicking sound heard while the engine is running.
They are relatively cheap, but depending on the vehicle, it can be a large job to get to them for replacement. Your friend may want to consider rebuilding the top end if the heads get pulled.
