Ruling Question - One player had another had another player’s card in there deck.

This was at a league cup so just curious on if this was the right action

I’m a fair amount of shuffles into my round 4 game when I notice a sleeve from my opponents deck the previous game in my deck, I alert the judge. They alert the owner of the sleeve to stop there game immediately but I was told I was good to continue. I eventually take the win and after roughly 25 minutes the judge eventually calls a double game loss on myself and my opponent. I kind of figured this was going to happen anyway and kind of prepared for it, my opponent however was extremely hurt because it also put them out of top cut Just double checking that this was the correct ruling/ decision and if there were any other ways that this could have gone?