What does he charge the client for this gig? Is it comparable to what you would charge? Or would you charge less? If you charge less simply because you don't NEED the income, then I would say you're undercutting him. But if you charge a reasonably similar fee and just bring better equipment, that's his problem not yours.
I'm in a somewhat similar situation as you. I own a systems integration business, but I still get asked to provide and run sound for previous clients. My fees have gone up rather than down now that I don't rely on the sound business for income. It helps weed out the people that I would rather not work with. My clients have gotten better, they appreciate me more, and the gigs are overall much more enjoyable. And if I lose gigs because my prices are too high, that's perfectly fine by me.