Can you cash a refund check made out to someone else if they sign the back of it?

No. A check can be cashed only by the individual to whom the check is made out (issued) to. No one else can cash it even if it is signed by the person to whom the check was given. If you try to do so, it is an illegal activity and you can be jailed for it. But, you can deposit that check into the account of the person to whom the check was given if he has signed the back of the check (endorsed it) and no one will stop you if you do this and it is perfectly legal.