The Attacking player can't cry that they wasted their moves to attempt the KO with a 10 damage attack.
It was their responsibility, along with the 'gatr's player's, to make sure that all cards were played correctly, not just their own.
I'll give two examples where it is OK to allow your opponent to screw up:
1.
Your opponent is playing Slowking. You play a Trainer. You want one, two...three seconds. Your opponent says nothing about using Mindgames. You can at this point continue to use your Trainers effect and not worry about him saying, "Oh! I forgot. Take that back, I have to flip for Mindgames".
Nope. The flip was optional and he had time to invoke it. Since it was not mandatory, you were not obligated to remind him of it, just to allow time for it to be used. You could not, however, have played so fast that he did not have time to invoke it. That would be illegal play.
2.
You are playing Erica's Dratini. Basic Pokemon can only do 10 damage to it. Your opponent is playing Wigglytuff. He's unfamiliar with the card, and so asks to read it. You let him. It appears he did not read it correctly, because, with a fully powered Wiggly that could KO it with Do the Wave, he keeps trying to put you to sleep for two turns. It seems he thinks that all Pokemon can only do 10 damage to it.
You may keep your mouth shut. The player is choosing a valid attack (lulliby) and is applying it correctly (putting you to sleep). It is not incumbant upon you to suggest that another attack would not only be better, but that it would work at all.
Note though, that if he had used "Do the Wave" and had put one damage counter on your Pokemon, you would be obligated to correct the misplay and put the correct number of damage counters on your Pokemon, KO'ing it.
See the difference?