I agree with Whale you were both in the wrong but your error was minor and done in good faith, whereas he took advantage of the situation.
As no vote or approval was given at any EC or OC meeting legally he is in the wrong as he doesn’t have the approval for any changes (just approval to repair the leak). I would suggest you submit a bylaw (written by yourselves) to the owner outlining his new responsibilities that you will vote at the next GM or he can provide his own bylaw to be voted on at the same GM and unless one is passed the improvements must be removed.
You could also advise him that if neither bylaw is passed you will be removing the offending pipes in your garage immediately after the GM. You are only legally obliged to leave the original pipes that do have approval.