I would love to see this match actually happen! I'll try to answer it seriously, although if they ever played, I'm sure it would be an exhibition with Bahrami doing his usual shtick. Slow motion points with no balls, and all that good stuff.
But assuming it's just one match, and they were both trying to win, I think it could turn out like Riggs/Court. Bahrami can do ridiculous things with a tennis ball. I don't think Serena would know how to handle the crazy spin and angles he can produce, because you never see anything like that in the WTA. To be honest, you rarely see anybody like Bahrami in the ATP tour either. Serena's serve would still be very effective, but he'd be used to the power she can produce off her groundstrokes. He still plays with the seniors, and those guys have not stopped trying to serve better, hit harder with new racquets, even if their legs have gone. I think he has the perfect game to run Serena all over the court, while protecting himself from doing too much running. I think it could be a case of a guy who has honed a really unique game all his life, against somebody who has no idea how to play against it.