It will be a dramatic change, even as a hybrid, so go carefully: lower tension, use the softer Lux strings (M2 Pro, ALU Fluoro) or other brands of softer polys (see my signature). Also, the cross should be a multifilament, perhaps even the Sensation he is currently using. My customers like Babolat Addiction in the crosses.
Why don't you knock it off with them negative waves? why don't you say something righteous and hopeful for a change?