What you don't do
Don't bring this up in the middle of a dangerous battle, when the player tries to use the power at a crucial juncture. (Then the GM rules it illegal, some PC bites the dust, and it's all your fault.)
Who do you talk to first? It's all got to do with personalities
I don't really think there's a correct answer, in the general sense, whether to speak to the GM, or the player in particular. In some gaming groups, just bringing this up at the beginning of a play sessions would be find. It very much depends on the personalities involved.
You are afraid the player might feel you "went behind their back" if you go directly to the GM - but all you're doing is discussing the rules of the game. Is the person just a little touchy, maybe?
If so, then maybe you do want to talk to them first. That's a social smarts decision you are going to have to make the call on, yourself. There are pitfalls either way.
(Other answers give guidance on speaking discretely with the GM. The rest of this answer assumes you would rather broach the subject with the player in question, first.)
If you talk to the player first
The trouble with talking to the player first is they might object to you telling the GM. Then you have to explain why you are going to do it anyway, and try to get them on board. (Because you don't want to let them talk you out of it.)
Set a friendly and cooperative tone when you broach the subject, something like, "You know that power we felt was too good to be true, we might have been right about that. It's a good power, but not quite as super as we were reading it as."
Objections the player may have to informing the GM
The "For the Good of the Party" Defense
If the player says that the power helps the party survive and succeed, remind them the GM is already complaining about it being hard to balance monster encounters with this power around. That means, you can expect harder monsters in the future if you don't "come clean." So if this power fails, the party will be in deep trouble.
The "You are just jealous of my awesome character" defense
If this is a "power gamer" who wants to have the strongest character, assure them their character is still plenty tough.
Under the current rules interpretation, his character is too much stronger than the others. Assure them that even with the new ruling, their character will still be tought and a great ally.
Make sure they know you are not trying to "stick it to them" but feel the game would be more fun when they get the balance right. Some of the other players need their chance to save the day.
The "You're on the wrong side" defense
The player may try to make it into a players-vs-GM thing, and that informing the GM is like betraying the party. You remind him the GM is just trying to get the balance right. You are actually helping the party by making sure the GM doesn't have to send bigger and badder monsters against you.
Whatever you do, don't agree to keep this secret
No matter the objection, you can just let the player know that, since the GM is bound to figure this out sometime, you really think it's best the GM is informed right away.
Encourage the player to bring up the issue themselves, and if they do, don't ever mention it was you who discovered the rule. Even if the player gives you credit, emphasize the player's role is reviewing the rule and coming to the new understanding.
If you can't get agreement from the player to inform the GM, just remind him (if your game system allows it) that you are Lawful Good and cannot deceive an innocent Game Master.