There is several things you can try:
1. Candid private talk with the individual. State you noticed that the dice rolls seem to defy statistical probablity fairly consistency and ask for a explaination. Then state you are implementing a policy of all rolls from *everyone* will be witnessed or will not be accepted. Unfortuately, in essence, you are calling the person to task on this so the person may not react favorably to the what you are implying.
2. If the person is rolling so called *hot*, state that you are amazed at how consistently good the rolls are and you want to witness this phenomenon first. Roll that puppy and lets see it....
3. Accept the person's declared rolls. If it is a hit on a monster, state the attack hit and allow them to roll damage. Record nothing and immediately go to the next person. Unless the person is oblvious, it should occur to them in short order that their rolls are being discounted by you. If they ask why, state you will discuss it after the session. If it is a saving throw, don't state the target DC. Let them state what their dice result is and then announce that they take X damage. Don't explain how you calcuated it. If asked why they took full damage when a lesser roll by someone else resulted in half damage, state you can discuss it after the session but the damage is what the damage is. Unless the person in question is dense, they should pick up that their rolls are ruled upon in an arbitrary and inconsistent matter. When the discussion comes up, state your concerns and discuss. Let them know that you don't intend to supervise their rolls (we are all adults here, I assume) but as long as you feel they are not being forthwith with the dice, your adjudications will continue to be arbitrary and inconsistent in response.
4. If the person appears to be cheating, roll your own dice and apply the result. Then announce the result. If challenged on it, state your concerns about biased dice and skewed probability and that anytime that anyone makes a roll you consider questionable, you reserve the right to make your own roll and apply it.
5. Accept the rolls and the cheating. Make sure that all encounters with multiple monsters have one more than the number of party members and that monster attacks the cheater's character. If a solitary boss monster, the most damaging spell or attack get directed at the cheater's character. If a activated trap or a unfortuate event can affect random party members, the cheater's character is the one that take the hit. Unless they are dense, they will figure out. If confronted on it, simply refer to their 'phenomenal luck' and that because of said luck, you felt they were the best candidate to take the extra damage/monster/act of god. (Actually, I like using this option the best with cheaters. They figure it out in due time and is stops the cheating right quick)
Anyway you slice it, there is no easy answers. Not a good situation to find yourself in.