It's skill fs. My friend lives from bets and all I can say that from tenis and a few tournsments betting he lives not bad at all.
In this situation, the fact that your friend is well versed in tennis and can make good money on it proves, and this proves that you can make money on it with the right approach. I have a friend who makes good money on boxing because he himself was a former athlete and this helps him in analyzing the favorite of the fight. He does not make millions, but it is enough for him to have a good vacation once a year.