Put Greater Dispel Magic in a (or two) Ring(s) of Counterspelling. Cheaper, and leaves the Contingency for other uses. There was a thread not too long ago debating the use of Contingency to counterspell things; if you do indeed allow it in your game (I would) I'd use it to make a Greater Dispel Magic attempt to counterspell a Disjunction.
Thanks - I looked at his character sheet and he did indeed have a Ring of Counterspells with Greater Dispel Magic in it.
So, unless he was doubly concerned on being dispelled, he would use Contingency for something else.