I played a guy in singles I strongly suspected of tanking a set to me once, but it was nothing like the match described in the OP. Also there was a full 3rd set so he wasn't actually risking that much as far as the match outcome goes.
First set, the guy was just going for broke every shot, like he just didn't care where the ball landed. No shot selection other than pound the ball as hard as he could. This was 3.0 guys tennis at the time so in an of itself this wasn't something too weird. He made enough to still win some games, but I ended up taking the set 6-4 without doing more than getting a few balls back in play here and there. Average point length was maybe 3 shots.
2nd set starts and from point one, his whole game completely changed. He's a different player. Now all of a sudden he's in complete control, hitting solid groundstrokes (relative to 3.0 or 3.5 level of course) deep and hardly ever missing. I can't get an edge anywhere and he's just not missing and he moves through the next 2 sets 6-0, 6-2, and all I remember is having to play darn well (for me) to even get those 2 games.
He was a computer rated 3.0, but my take was that he was on a team that won their flight and went to playoffs year after year, and despite improving over time he didn't want to get bumped and miss out on that rec tennis glory.
This OP description though leaves me very unconvinced. He could easily just be a guy who just undergoes fluctuations in confidence and level of play (which could also explain why he's a 4.0) rather than a tanker. He's self rated so he also could be just starting to play tennis again after a layoff which could also explain the fluctuations in play. A real tanker wouldn't need to let it get that close at all. 6-4, 7-5 is like never going to generate a strike, no need to let it go to a TB.