Your friend is going to have to make a deal with the team/league if he wants to sell his photo without opening himself up to the possibility of legal action. There are a few principles at work here:
If you sell a photograph of a person without getting a signed model release, the subject can sue you for a portion of the proceeds of the sale plus punitive damages. It doesn't sound like your friend has one. The pros (SI shooters, etc) are covered by the agreements they sign with the league and the agreements the players sign with the league. These agreements typically strictly limit how the photographers can use the images they shoot and who they can sell them to, and at what prices, etc., as well as spelling out liability issues.
You may not realize it, but when you purchase a ticket to attend a major sporting event, you are legally agreeing to a bunch of terms and conditions governing what you can and can't do at the event. Typically the high points are spelled out on the stub somewhere, along with a reference to the complete text of the agreement. Shooting photos of the game for profit is pretty much always specifically forbidden.
Trying to sell the photo to the team/league would avoid the legal problems, but that is a VERY tough path to go. For every working pro, there are at least 1000 photographer wannabes, and many of them are actually talented. You need excellent sales skills as well as lots of talent and deent gear to get anywhere. Selling to a third party would be easier, but more risky legally. Your friend may not get caught, but if he does, the lawyers will pretty much be able to crush his testicles in court.