The tennis has lost its grace when people like you have to be put in their place with yet more rules addressing all sorts of foul practices. The rules about the tennis material are very specific as to what is allowed and with a good reason. Once upon a time even the spaghetti strings were banned as to not alter the fine margins within which the sport is played (they were considered cheating), and nowadays even direct rules are not imposed.
Every existing rule regarding the tennis material has the clear intention of keep it within tight tolerances and not allowing for anything outside of the natural deterioration as a result of playing with it. You may repeat till the cows come home that there is no specific rule, but the spirit of every existing rule regarding that is clear.
On a separate note, he does it with the clear intention to gain an advantage. The parallels with other sports clearly also indicate that he thinks that it does. It doesn't even matter if it really gives him advantage to condemn the practice on principle. Cheaters are going to cheat and people that indicate that they are ok with it are also cheaters.