Broach the subject outside of game time
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 DM rules it illegal, some PC bites the dust, and it's all your fault.)
Who to talk to first? It's all about the personalities
I don't think there's a correct answer, in the general sense, whether to speak to the DM, or the player in particular. In some gaming groups, just bringing this up at the beginning of a play sessions would be fine. 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 DM — 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-skills 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, 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.
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
Over the past 35 years, I’ve heard a lot of excuses for trying to hide rules information from a DM. Here are a few common themes, and how you might respond to them.
The For the Good of the Party Defense
If the player says that the power helps the party survive and succeed, remind them the DM 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’re just jealous of my awesome character objection
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 tough 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-DM thing, and that informing the DM is like betraying the party. You remind him the DM is just trying to get the balance right. You are actually helping the party by making sure the DM doesn't have to send bigger-and-badder monsters against you.
Don't agree to keep it secret
No matter the objection, you can just let the player know that, since the DM is bound to figure this out eventually, you really think it’s best the DM 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 DM, just remind him that you are Lawful Good and cannot deceive an innocent Dungeon Master.
The biggest downside to this approach is that you reveal yourself as the person who informing the DM of the issue.
As a teenager (ages past) I saw cases where the player of the “nerfed” character tried to turn the rest of the players against the “informant.” These typically didn’t amount to much, since the other players don’t want their characters overshadowed due to a rules misinterpretation, either.
(It’s always possible for adolescents to eject a member of their peer group, under this or that pretense. But in that case, if it weren’t a disagreement about rules of a game, it would be something else very soon.)
Stick to your point about wanting the game to be fair and balanced for everyone, and resist making the argument personal, and no fair-minded group will blame you.