Inaccurate checkmates? So, in a tournament, my daughter recently had a case where her opponent made a move and declared checkmate. She acquiesced, shook his hand, and then realized it wasn't actually a checkmate. They continued the game until she actually checkmated him, at which point he said, "Well, you shook hands.." and claimed the victory in the tournament.

I can see both sides of this, but I am really not sure what the official ruling in a case like this would be. Anyone have any thoughts?
Checkmate ends the game - there is no agreement/concession involved. If it’s not checkmate the game has not ended. And a handshake is not a move which needs to be “retracted”.

If the error was only realised significantly later, then maybe it’s a different case. I can see that an arbiter might deem the handshake to be a resignation. But that’s not the case here: they just played on and did not mark the scoresheets.

Your daughter does not need to be an expert in the rules (which cannot not be exhaustive in any case) but does need the reflex to call the arbiter or tournament director in any kind of dodgy situation.