Test the dice and use a dice tower
Most of the dice people use are uneven and the insides aren't weighted properly. It's the way the dice cool off when their made. The way they are made is the problem. Their not made with precision and randomness in mind.
You can test your dice to ensure they are balanced. I believe these seeing the dice being tested on the videos helps more than describing it. Daniel Fisher's youtube channel is a good resource for dice testing videos.
You can take salt (keep adding until a die will float) water and see if the die is uneven as it will favor a certain side/sides more often.
I just realized that your player is using metal dice. Contrary to popular belief they can be made unbalanced. I've tested a d20 from one of my player's (it was a mini metal die). It's a painfully slow way to test as you can't reasonably use the float test. It had a notably uneven amount of high rolls!
If the protruding clip mark of Gamescience dice is properly filed or cut off they are very balanced dice. Here is a good article called How True Are Your D20's.
Col. Louis Zocchi (the owner of Gamescience dice) has stated that the type of surface (felt, metal, plastic, wood, etc...) you roll on will affect the randomness of the dice! Another plus for having a dice tower.
I've shared this information with players. They liked the information and actually enjoyed testing their dice in salt water. For ultimate peace of mind try to get clear dice (it's the difference in how the clear dice are made, any brand should work) without air bubbles, but more importantly have everyone use a dice tower! Get one or two for the group. It has been a huge blessing to my rpg & boardgame groups!
Here are two I recommend through use.
The second is hand crafted and felted! A little pricey... but nice!
The Romans used these to make sure their gambling games were fair! Even accusations of cheating in those days could get you killed! Dice towers save lives! PC's included.
Using dice towers has taken out all & any animosity about dice rolling from all the groups I've played with. When I was younger I figured out how to manipulate dice by how I rolled the them, as soon as I figured it out I made a point not to do it as it ruins the game, but it's possible someone could be doing it without realizing it by the way they roll. The dice tower fixes this.
I generally roll very well and my friends know I hate cheating like they do. Using the dice tower let's everyone be at ease.
"We may throw the dice, but the LORD determines how they fall." Proverbs 16:33
Dice are biblical! ;-) Let's just try to make sure we're not overly influencing the roll!
I hope this information will help.
