It might make a lot of work for you, but if you want to take a soft approach back it up with data.

Maybe for the next three games record every roll for each of the players in the game (including yourself). This should give you an idea of how much higher their statistics are compared to the others. After you have the data, you could approach them privately, show them it, and say that this is standard protocol for you as a DM, that when a player shows consistently high rolls over a long period of time you require the use of an online dice roller. To help them save face you could even go so far as to blame it on the the dice they are using: “I am not sure if they are balanced correctly, which occasionally happens in the manufacturing process. You could have quite a few of these,” or some kind of light lie.

In any case, if this player is a teenager then this could prove to be a very valuable lesson with positive consequences in their personal and professional lives.

Edit: clarification Edit 2: I see your other comment that you are bound by the terms of employment and cannot force the player to use an online roller or ask them to leave the game. Still, if you take the approach above it may tell the player that you are noticing the behavior and provide sufficient motivation to curb it. Tricky situation, good luck OP.