Is it okay that every once in awhile my friend beats himself with a 2x4 to keep himself in line?

You should only be worried if your friend is doing serious and/or permanent physical damage to his body with the 2x4. I use a belt, and, small whip on my *** when I feel I deserve to be punished. I know of others who do similar things to themselves, so it is not that unusual. 

He needs mental help.

  A 2x4 what?