My dear, it's you and @Starspangledrodeopony who don't know how tech works. The error when porting numbers over is a common one.
People are saying burner phone (and again, why would he reply to a message that doesn't exist on his burner phone?) because the message isn't in his original phone's thread. If it were a dual sim, it would still be in the thread on his original phone, so that's immediately ruled out. Message history in stored on your phone, not on your sim.
He could've just deleted it, but in the first place it's unlikely he texted her with his 2nd sim.
When you reply on a dual sim, it replies with the number you've selected as your main number for Messages in Settings – and absolutely nobody with common sense changes that on a day to day basis, not even for an affair, plus sometimes you have to restart your phone to get that setting to line up.
Most people manually adjust the sim for individual messages. On some phones, you can manually adjust it to Sim 2 for the entire message thread with a particular contact/thread (i.e. with his wife, which is hard to do accidentally).