Honestly, If it were me, I would allow him to keep one visual morph (male or female) of his choosing. The rest I would keep/sell. If I was housing, heating, caring for and feeding (I know you said he buys the f/t rats however still my chore) the animal I would want to get some compensation for it. I think your friend should find that more than fair as if it were not for your generosity, they would not have a snake at all.