First of all either you or the friend could be wrong in the determination of the sex. Can a reliable 3rd party take a look at it? Is this buyer local? If you are wrong and don't refund, the bad publicity that you will get won't be worth it. If you refund his money you can resell the snake after determining the sex properly and avoid having an issue at all. Or if it is male maybe he would be willing to keep the snake and accept a partial refund. I wouldn't just refuse to attempt to resolve this if I were you.