I don't believe you.
You apparently lost that much GSP in such a small amount of time, when the average match would have been 7 minutes. If he played for 20 minutes, then he would have only played a few matches. Factoring in the time it would take to match up with an opponent, the match playing out, post-game menu's and then searching again. I'd estimate he got to play 3-5 matches, tops.

Your GSP loss would not have been that drastic after such a small amount of losses. After sitting comfortably at elite for some time, with multiple characters - I've found that I generally only lose around 10k - 30k GSP per loss. The only way I could see you losing so much GSP is if you were new to the game (or Bowser Jr) and your GSP hasn't settled in yet. In which case, your win/loss ratio shouldn't even be a concern. Speaking of which, how did your win/loss ration get ruined after only a few games?