Do you think that your friend will agree to signup via your referral link at a sports betting website that you have chosen?
Why don't you just create and fund your own sport betting accounts and just copy all his bets.?
You will make more money that way(if he is really that successful at betting) rather than relying on referral commissions,which aren't that high.The risk would be a little bit higher,because you will be using your own money,but the profits would be higher as well.
Are you really sure that he is so successful at sports betting?I know about a ton of people,who are bragging with their fake wins at Facebook and other social media platforms.Screenshots can be faked by using Inspect element.Photos can be faked by using Photoshop.