I don't know the class, or the errata, so I'll only have a general opinion:
What I hear here is that the class is considered overpowered enough to nerf it in the errata, and that indeed the character has been quite good at what he's doing, maybe too good.
I'd say that you should go ahead and implement the errata and bring him back to normal power levels. If he's in it for the interesting choises the class presents, he won't be bothered that he suddenly is no better than the rest. If he's in for the power, he should accept that overpowered choices can get taken away.
It's not punishment, either. It's as if a father decides to give each of his children 50 quid, only he inadvertantly gave one child 100. He realizes it, and asks for the money back. It's not punishing the child, it got more than it was due, it was an error, and will be fixed now. In fact, you could say that not taking the 50 quid back would be a punishment for the other children, because they got less.
Should the father give everyone another 50? Anyway, shouldn't the child that got too much money not give it back by himself?
Personally, I like powerful characters. I guess I am powergaming to some extend (though the characters still have, well, character). But If the DM thinks that something is too powerful, I won't throw a fit because of it. I might discuss it and show my point of view if I think that it isn't, actually, too much, but if I see that it is too much, I will gladly exchange whatever it is for something else. Happened a couple of times so far, and it never took the fun out of the character.