Back in September, my son lost a match 2-6/3-6 in a final and he blamed the loss to his opponent's 2 bad calls. I showed him the stats that his opponent hit 10 more winners and 20 less unforced errors than him. I told him you lost the match because your opponent played much better than you. Taking out 2 bad calls, your opponent would still hit 6 more winners and 16 less unforced errors and you probably would win one more game. BTW, he learned from that loss and beat the same kid 6-4/7-6 in November and 6-3/6-4 a week ago.
When my son plays a known cheater, I ask him before the match that he should call umpire after 2nd bad call. I also advise him try to aim a foot or so from the line and don't give his opponent many opportunities to make "close calls".