Can a player concede in response to a trigger that would win them the game to deny the opponent the win?

A player has the right to concede a game at any time, as stated in CR 104.3a: "A player can concede the game at any time." When a player concedes, that player loses the game immediately (CR 104.3b). The game ends on the spot, regardless of what is on the stack or what triggers are pending.

This means that if a triggered ability is on the stack that would cause a player to win (e.g., the final poison counter trigger, a "you win the game" ability from Thassa's Oracle or Test of Endurance), the player who would lose can simply concede before that trigger resolves. The game ends, the trigger never resolves, and the opponent does not receive the "win" — the conceeding player still loses, but the opponent is denied a clean victory in contexts where that matters (e.g., tournaments with prize splits or opponent's win-percentage tracking).

Importantly, this is not considered slow play or unsporting conduct in most tournament contexts — concession is an unconditional right. However, a concession cannot be made conditional (CR 104.3a note: concessions cannot be offered in exchange for in-game actions), and once made it is final.

Example: Your opponent controls Test of Endurance and has 50+ life; its triggered ability goes on the stack at the beginning of their upkeep. Before it resolves, you say "I concede." The game ends immediately. Your opponent never wins via the trigger — you simply lose by concession. In a tournament, this affects your opponent's game-win percentage but not their match result.

Citations: CR 104.3a covers the unconditional right to concede; CR 104.3b covers the immediacy of the loss upon concession; CR 104.2 covers how a game ends when a player loses.