The menu event script for this is mev_battle_escape, you'd just need to remove the special case for trainer battles.
Though you might need to take further measures to prevent the player from immediately ending up in a rematch, cc_battlestart_trainer saves the player's position after they've entered the trainer's vision range. So you'd either need to keep track of a previous position somewhere so you can deposit the player out of harm's way, or maybe give the player a temporary invincibility to further battles after they get away (and while this countdown is ongoing obj_npc_trainer's User Event 2 code - which is where they look for players - will ignore them)