whist not being ideal to drink on a regular basis, due to his body getting a higher and higher tolerance to caffeine, therefore making the whole point of it useless. it probably isn't as bad as some of you seem to think it is on a caffeine basis, if we are talking sugar then yeah, it is a horrendous amount.
caffeine is fine, so long as your not an idiot and taking crazy amounts, the body can handle a lot. theres some people who take 6 shots of espresso in their coffee in the morning every day in america / canada and they have not died yet. otherwise coffee places would be getting sued for basically selling poison.
as said before the body can take a lot of damage, whilst a vodka red bull is not healthy, it probably isn't going to kill you.
i would strongly recommend keeping caffeine to a minimum though, because of how the tolerance levels build up. the reason why regular coffee drinkers, drink a lot of coffee. also possible side effect are headaches, migraines, blurry vision, etc from regularly taking a lot of caffeine.
these energy drinks are not going to kill him but they are going to stress his body and he is going to suffer for it, you could say similar things about alcohol, look at how alcohol, stresses the body, damages your liver, etc. drinking 6 cans of lager a day isn't going to kill you but it's not healthy either.
having the odd can here and there is perfectly healthy, but i would not be taking 4-6 a day, your going to be getting less positives out of it by drinking so much. also there is a reason why your drinking that much, your tolerances have undoubtedly built up and you need that many for the same effect 1 can used to give.
i would suggest you drink 1 can a day max, or at least switch to a sugar free version.