McGonagall's Disloyalty

[madaboutsteps at barclays.net]

No: HPFGUIDX 29937

 In answer to Calypso's message about what Klawz wrote
 Klawz wrote:
 "What say ye to the idea of Prof. McGonagall being the
 one who betrays Harry? <snip evidence>"
 
 Then Calypso wrote:
 "Why didn't she stop The Dementor's Kiss on Crouch? Well, I assume 
that a 
 Dementor is something that not even a witch or wizard wants to get 
in the way of." 
 
  
But I think I would have to disagree with this, because if you may 
recall in PoA when Harry fainted in the train that Lupin told the 
Dementor to go and when he did not Lupin "muttered something and a 
Silvery thing shot out of his wand" which made the Dementor leave. 
Does this mean that McGonagall could have stopped it? or do you think 
I have misinterperted what happened?