doctorbadwolf
Heretic of The Seventh Circle
Any rule they have established that the game will be following.What rule is he going to break in order to be cheating?

"We aren't using any houserules for crit damage, we're just going to run them RAW, if everyone is cool with that."
"Yeah sure."
later...
"Oh that's a crit. Ser Bertrand's lance slams into you like an avalanche as he charges you at surprising speed. You take....12 plus strength is 16, plus..." rolls a d12 "6, for 22 damage."
"Wait, we agree we aren't doing crits that way, we're running them RAW, right?"
Now, if the DM says, "Oh, right. Sorry, Forgot. So it's 6 plus strength for the die I already rolled, and lets see what the second die comes up" then they aren't cheating, they just forgot a thing.
If they say, "Yeah I don't want his crit to be weak because that would feel weird for this moment" then they are cheating. They're breaking a rule because they feel like it, because it's inconvenient.
If they say, "Hey, it feels not great for crits to sometimes be lower than a high damage roll on a normal hit. Any objection to using the max die plus a bonus die crit model?" that is also not cheating, they've just openly changed the rules from that point forward.
Changing the rules in a way that screws over the players can be an abuse of the authority granted by the players, and players should absolutely push back against that and remind the DM that this is a voluntary group activity, not a gift the DM is granting to the lowly PCs who should just be grateful and shut up, but I wouldn't personally call that cheating. I do know plenty of people who would, however, call that cheating.