We had a similar situation at my gym. We had 2 guys start at around the same time.
The one guy complained about having to be in the beginner classes because he trained MMA for almost 1 year with some 'pro fighters'. He was pretty athletic and had a good body type for BJJ and Muay Thai. He was constantly complaining to anyone that would listen that he shouldn't be in the beginner class. He had a lot of bad habits to break and most of the time he was just a touch too stubborn to listen.
The other guy started and was a 30 something year old guy with an IT job. He was shorter, overweight and had no previous training. He never complained about being in the beginner class. The first night he graduated out of the beginner program and was allowed to roll he was ROLLED UP by a 15 year old 95 lbs kid. But he kept training, lost weight, came off his blood pressure medication and did a tournament.
It just so happened that the two were in the same bracket of a double elimination tournament. The second guy with the IT job just absolutely abused the first guy, he was up 15-0 before arm barring the guy. In the finals they met up again and the second guy was up 12-0 and arm-triangled the guy again. I was never so happy to see a 'team mate' lose. The first guy was ultra cocky, had invited his girl friend and parents to the tournament and it was great to see him knocked down a peg. I was also really proud of the second guy. Later we were talking about the tournament and he told me that he went in to that match really scared because he just didn't want to lose to THAT guy. He would have been fine with losing - just not to him. LOL
The first guy no longer trains.
The second guy got his Blue Belt in DEC.
that was poetic justice