If you're going to be housing both adults, taking time to feed both adults, taking time to incubate all the eggs and then care and feed all the babies (including the possibly dirty work of having to dispose of any that don't make it), then I would say it's fair that he gets to keep his snake, and one visual baby. Maybe the pick of the litter sort of deal?
Do you guys have a contract of any kind? The only reason I'm asking is what happens in the instance that the female becomes ill, or suddenly passes away? Or she becomes egg-bound during the breeding process. Or needs emergency treatment of some kind?
I'm a BIG believer in contractual agreements, especially if you're entering into a business relationship with someone, even if they're a friend, and even if it's a quasi-business relationship.
EDIT: a contract would protect him too, from you taking his snake or simply selling off all the babies, etc. I'm not saying either of you would cheat the other, but it's just nice to have it all buttoned up and legal. That way everyone knows what the deal is, and no one's left hanging without some sort of way to recoup their losses.