"Because I can" or "Because it's funny" are more than enough reason for most people. The fact that the hacker refunded all the customers, then emailed them to warn them that they were using a terrible app actually sounds like a pretty tame troll to me. If the hacker was truly hired by the competition to act maliciously, they could have done far, far worse.

Your friend should take this as a lesson instead of trying to deflect blame to their competitors.