Are you imagining that DMs wait until the players have their PCs do something suicidal and then say, "Gotcha! Your character is going to die now. Muahahahahaha!"? Because in my experience the DMs, including myself, inform the players in advance and if the players then make the decision to do it anyway, it's no one else's fault. They were fully informed in advance.
Good for you, but I am saying based on stories that I've heard that DMs do play "Gotcha! Muahahahahaha!" and do not inform players in advance. No, I don't think this player was fully informed in advance.
In general, not knowing everything about the outcome as a player is to be expected. The consequences of an action are not necessarily obvious to the player. He's usually operating with limited knowledge of the situation. If the player doesn't know the 100' deep spiked pit trap is there, then the GM is not obligated to tell the player walking forward is potentially suicidal. But on the other hand, I've also heard stories of "the floor is lava" where the player was standing within 5' of illusionary lava without feeling any heat or taking any damage, walked forward and the DM was like, "You've stepped on lava! That's instant death no saving throw! MUAHAHAHA!"
The situation here is the player has a metagame expectation based on his knowledge of the rules and it seems like the GM has not informed the player that under his rules, falls of an arbitrary height set by the GM are always instantly lethal. It seems highly unlikely that the player made the decision to break free from the grapple with the dragon knowing ahead of time that doing so would be instantly lethal, but rather expected that the fall was less dangerous than the dragon.
In my game, sure, I stop and explain the rules for falling and explain to them that while the fall isn't instantly lethal and I will dice for it, the average damage of a fall of that height is around 93, but that there is about a 1 in 6 chance that they take over 230 damage and that they will be making DC 15 Fortitude saves to avoid traumatic injury about 5 times in 6. In this game, I doubt such information freely flowed with a "Are you sure you understand the rules?" check.