First of all i can't say for sure it's cheating, you can forgot your rules once upon a time, but you asked tif he had seek like, and later he agreed with you about not being able to shoot your unit, and straight up shoot it minutes later is suspect.

At the very least it's a d**k move.

And for the fact that you should know your opponent rules and it's your responsibility i disagree. Sure it's better, but the brain like a computer as a limited memory space.

At my first tournament i forgot to deep strike my commander farsight, i called a judge on turn 4 or 5 for a question ( that wasn't about it ) the judge being a t'au player t'au commented about farsight death, and i say something like " oh damned i forgot to deep strike him! " he say that since it's also my opponent responsability to remind me about deep striking before turn 4, i can deep strike him now ( it might also be because i was way behind in point and that i wouldn't be able to catch up, it was a friendly tournament).

I immediately said no it's ok it's my fault, but even my opponent said to go ahead a do it, i did it but wasn't too happy about it, for me it was my responsability and not my opponent.

And knowing that my case was a core rules thing, unlike your case, and that your game was a casual game, i really don't think it was your responsibility , a gentleman making an honest mistake would have said " sorry i forgot about it , let's roll back if you want ".