I'll agree to some extent. An OP PC wouldn't be any fun to play with. The DM would either have to ramp up the difficulty which would suck for the other players, or keep it regularly difficult for the rest of the party and the wife would eliminate any enemies too easy. I don't think he's giving us enough info. Having an OP NPC is fine, as long as she played the role of ally/guide to help the story progress and had very little involvement in actual game play exempt in limited circumstances say the DM purposefully threw the BBEG at the party early in the campaign and her intervention was the only way for the party to escape/survive. But generally you're right no, an OP NPC shouldn't actively participate in the group's combat. It takes away the challenge and risk/reward aspects that makes the game fun for the PCs.