Cool! He's spent 3 feats to get there (Leather Prof, Hide Prof, Imp. Armor of Faith) of the 4 he gets for 6th level. That's not an insignificant sacrifice.
He also needed a Str 13 and Con 13 (for Hide Prof. feat), and since the class requires neither of those stats (Avenger needs Wisdom primary, then Dex or Int secondary), his PC spent some significant point-buy to get there. So his PC base stats (pre-race, 1st) were 16, 15, 13, 13, 10, 8? That's not a great distribution (and it means he put a 15 on his primary stat: Wisdom, which sucks) so he made sacrifices there too. What race is his PC?
I'm guessing he started this PC at 6th, and so didn't "feel the burn" of this build.
Bottom Line: this guy sacrificed his attack roll *and* three feats to get to this high AC. Is that *really* a problem?
I guess the answer is yes, because he has a class feature that more than makes up for his "sacrifice". I seem to recall reading some other thread where someone had said that the 2 dice rolls you get from the Avenger class feature worked out to about +3.34 to hit statistically? I know it was something just over +3 so bottom line is he still hits just as often as any other striker if not more since it is assumed that you will have between a 16-20 primary stat and he has +3 above that from rolling twice.
Now that you mention it I should probably check to see if he didn't start with a 17/13 to get his 20/16 by using his level 4 bumps and then having enough points for the 13/13 Str/Con. You are correct in that he did not have to "feel the burn" of the build. IIRC you only need 13 Str OR Con for Hide, not both. No books/rules handy from work.