He will end up with pierced primers. In fact it will blow so bad he will not be able to tell there was a primer in the pocket. More likely than not it will also cut the firing pin spring, as well as the firing pin. I saw this happen with a friend that did that with H-335 and Remington small rifle primers (the ones that say do not use in .223 Rem loads.) Cost him a firing pin, and a spring in his AR.
If it were something like .22 Hornet then it might be a remote possibility that magnum pistol primers with light loads would work.
Unless he wants to change out firing pins, and the spring I would suggest pulling the bullets. If he woks slow with the decapping pin then the primers can be salvaged.
If he were shooting Trail Boss small pistol primers would work. as well as if it were low pressure cast lead loads. Other than that I would not do it.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.