It's not necessarily a problem, and no one can tell if it's a problem beyond yourself and your players
Even if she reliably gets roll results in a restricted range, regardless of the reason, that's something that you can account for when setting DC for a challenge. But even that would only be necessary if the pattern of her roll results is making the game less fun for your table.
Really, it's only marginally more scale-tipping than all of the features and abilities PCs get. The game is about the PCs, and they generally have more options than NPCs and monsters and are typically expected to win (at least in modern-edition D&D games).
If you do think it's a problem, there are many possible ways to address it.
At the broadest level, this player is either cheating in some way or she systematically does something which narrows the range of outcomes on her d20 rolls.
If you think that she is cheating, then ultimately the solution will involve frustrating whatever mechanisms she is using to cheat. There are many ways to go about this, some of which will only apply to specific methods of cheating and some of which will apply generally. If she has developed a master's technique on rolling a fair d20, for an example of a specific cheating method, then preventing her from rolling her own dice would be the only sure way that she's rolling "fair".
As an example generic method, you could try reversing what results the die shows (so rolling a 1 becomes equivalent to a 20 for her, a 2 a 19, and so on). If she suddenly starts rolling a lot of results in the 1-5 range in a reversal of her current pattern of outcomes, it would be a fair indication that she is doing something to manipulate her rolls.
If you think she is not cheating (which, in my opinion, seems more likely based on the description in the question), then the most likely explanation for her unusual outcomes is that she does some things in a very systematic way and so similar starting conditions produce similar results.
For an example of this, some players like to play with their dice and have a preference for a die showing a specific value. If this player usually arranges her d20 so that it shows a 5 between rolls, and then has a very specific motion when rolling (like a gentle toss upwards with slight rotation of an always-similar direction and magnitude; this is something I've seen at the table), then the trend should be fairly easy to break. You could ask her to use a d20 of a different size, or have her roll with her off hand, ask her to "shake" the die more thoroughly in her hand before throwing it, have the d20 show a different number before she picks it up to roll each time, or any number of other things. Breaking up the systematic parts of the sequence should break up the narrow range of results.
In any case, it's not unreasonable to ask her to do something differently if you think these outcomes are damaging the game.
You don't need to know exactly why this phenomenon is occurring to conclude that it's harming your game (or not harming it). If the outcomes are really this skewed, this consistently, and always in the same direction, you can address it is as undesirable outcome instead of an undesirable player behavior. It's not unlike restricting classes or feats due to balance concerns.
If this player really is just fundamentally lucky, then changing up how she rolls her dice shouldn't derail her high-rolling streak. But with pretty much any other explanation you can expect some changes to her method to change the distribution of her roll results, and it's not about her or anything she's done "wrong". It's just a meta-balancing concern, and with a focus on keeping the game fun I think you can make that clear.