I'm sure coinmine know the account holders email and hopefully had ask him to split up their hashrate onto other pools. I find it highly unlikely that this person does not know what they are doing. One does not simply "have" 25Ghs lying about without knowing their impact on the hashrate.
What does it matter if he splits his hashrate? That only hides the problem, because it's still only one entity controlling a third of the entire hashrate (like someone here wrote).
Miner himself can't do anything harmful, only pool op can because it's a pool who is constructing a new block to mine, not the miner.
Well, he can always switch to solo mining with that kind of hashrate and construct his own blocks, no?
on 0% investment 
(excluding the actual hacking thing)
