I had a similar experience once, with a young man that I had been training since he was either 10 or 11. When he was about 13 his mother started dating a guy who had a brother who boxed in the police olympics, so he started taking him to the gym. This went on for 3 or 4 months, the mother and the guy broke up, the cop quit being interested in taking her son to the gym, so we picked up again. The young man was sluggish, no snap in his punches (and he had been a terrific puncher), no snap in his legs and it was like he couldn't hear me, just ignored everything I tried to tell or show him.
After a half hour or so of this, I asked me what was wrong. It turns out that his erstwhile 'trainer' had told him I didn't know what I was doing. I had been teaching him some things- ways of working to the body, using footwork to draw punches and create angles- that I picked up reading old boxing books and watching old fights that you don't see any more, much...and the new guy didn't understand them. So he told the young man they were foolish, and that led him to distrust me.
So we sat down and had a long talk, about faith and trust and credibility. I pointed out to him that I had told him up front that a lot of people wouldn't understand or recognize those moves. I reminded him that I had told him which fighters I had stolen them from, and that I had broken them down for him as he learned them: put your foot here, shift your weight, this is what you are trying to accomplish, and so on. The other guy had just said "that won't work." I noted that his uncle, a former Gold Gloves champion thought he and I were doing real well, that another friend of mine, also a gold gloves winner with 10 pro fights and hundreds of rounds sparring with Emile Griffith thought he was doing real well. We established that I am too cheap to be going around bribing people, and there was no trouble between us that would lead me to be deliberately trying to get him killed.
We let it go for the day, and I told him to call me and let me know if he wanted to box the next day. He did, and he was the same sharp, hard working, easy to teach kid he always had been.