How can a player lose experience?
Solution 1:
According to the Powerplay Manager Guide (I could find the same explanation on both the Hockey and Basketball pages):
Every manager gains experience automatically by playing the game regularly. For each day that you log into your team you gain one point of manager experience. For three days of inactivity you lose a point. Until you get to 11 points you don't lose points for inactivity. Once you reach this mark you will not drop below it.
(Emphasis mine)
So, your friend lost 2 experience points because they have not logged into the game for at least 6 days.