I noticed after the last lead someone reneged but it actually helped me so I just let it go
Most Euchre rules leave it the opponent (you in this case) to call the renege. They don't explicitly identify it as optional, but neither do the penalty sections go on to say what to do if you don't call it (i.e., there's no penalty for not calling a renege).

For instance, from these rules:


  the opposing team is rewarded two points if it is caught


and


  [unintentional reneges] are still callable by opponents as reneging


(emphasis added)