Truthfully if there is that much of a difference between the defenders and the 'squishy' targets AC they deserve to be beat on. You want your defenders to have an ac not all that far from the softer targets to make it much more viable to attack the defender instead of ignoring them. As far as the Avenger having high ac, I find it interesting that people complain about Avengers but not Swordmages, who have their Primary stat boosting AC as well as a class feature providing the same boost to AC as the Avenger.
Unless they're a deva in which case with 3 feats they get the +2 back from using a 2-hander, or +2 extra using a 1 hander, and it persists even if they drop, they also qualify for hide easier and as they have Intelligence as an attack stat they're likely to have a higher base AC before the warding is applied anyways.
As far as the Swordmage being a Defender and the Avenger a striker, the Avenger is a Striker/Defender who is based around going toe to toe with the big guy while the rest of the party deals with the rest of the enemies.
It seems to me the biggest bang for the buck for an Avenger is just taking leather armor. +2 AC (at least), one less feat, and no stat pre-reqs. The OP's player missed a trick, methinks.