Either you're using different definitions of 'high' than I am, or you're not using point buy. Even at 25 PB, Buying into Str, Dex, Con, and Int means either all four are mediocre, or he's dropped other scores below 10 to compensate...less than 25 and the problem gets worse, and fighters can't afford to shed Will save points if they don't want to be a liability. 14 Str, Dex, Con, Int, and Wis runs 25pts, and gets you no bonus higher than +2 on any stat.
'If it can't hit you, it can't hurt you' seems to be the philosophy that the fighter-builders of my aquaintance use.
So, having a low con? Only a problem if the monster can hit you. You've got the Attack Bonus to spare, so Combat Expertise (which seems to scale in PF) is a good choice. With Armor Training, having a high Dex becomes useful - it counts even in heavy armor, at higher levels.
That's my point in a way, because it goes both ways 'if you can't hit it, you can't hurt it'. The average melee monster has more Strength, a higher attack bonus, and more hit points than any fighter, so they can better afford to shed attack bonus - either for extra damage via Power Attack or extra AC via Combat Expertise. It's trying to beat said monster at its own game, and losing the numbers race. The feat also scales horribly, for instance - at +1/-1, increasing by 1 every 4 levels, and is only usable when you're making melee attacks. If an extra 1-3 AC is making a character unhittable, he's already outclassing his enemies to such a degree that he doesn't need the boost.
It's not just fighters here, but a side effect of hit points in general. The same reason why direct evocation damage is the worst of a wizard's bazillion choices for spells is why a fighter sacrificing damage for defense is a poor choice - hit points are binary. You're either at positive hit points and 100+ fighting effectiveness, or at negative hit points and helpless or dead. Whittling down their HP only gives them more chances to hurt you, even if you're reducing the odds of any one hit landing, so your best tactical choice is to either burn them out as fast as possible (for mundanes) or bypass their HP entirely (for casters).
Your specific game can of course be different - that's the whole point of statistical deviation, and it can't account for extraneous factors - maybe your DM likes using mobs of weak enemies, or 'lurker' type monsters, or (since you mentioned comboing archery and CE, which is illegal) is just making mistakes. From raw numbers, the guy in your group might be effective now, but he would be even more effective without CE.