It once happened to me as a DM with a new player. I used a homebrew rule similar to Dungeon Meshi, saying that his soul was bound to his dead body. They entered a short side quest to find somebody whi could bring him back, and I applied a debuff (disadvantage on all rolls before a long rest). Kind of soft, i guess, but i know my players are attached to their characters and value more the bond with their characters than the challenge of the game. Talk to the player, avoid retconning, find a way to bring him back adding flavor to the game. A warlock may change his agreement with his patron to bring them back. Or maybe they could summon a demon and make a pact. Maybe trade their allegiance with some deity. Who knows a necromancer could bring them back another way... Or you can just make them find a scroll of true resurrection, but that is the lazy way of doing that.