On an aside, my friend is a full time sports better who has an extremely high ROI during football season on one of those Fantasy sites. He has difficulty making any money on other sports. I wonder why.
Variance. Unless your friend has sustained this performance over a number of years, it's probably just variance.
Most successful long-time sports betters are not playing on a level playing field. In order to beat the vig over the long term, a successful better must be privy to information that is unavailable to other betters, or at least the majority of betters. This either means a novel analytical technique, which is rare, or inside information.
The number of people who simply watch games closely and are able to discern information that allows them to bet successfully on future games over the long term is exceptionally rare in the real world.
