He can probably use whatever pool he wants
Sure he can, then he's part of a fairly large issue plaguing Bitcoin right now. Unfortunately uneperienced noobs like yourself are here for quick easy money, see the fancy splash screen at GHASH and join them. Little do you know, that very same pool keeps amassing near 51% of the hashpower and has tried to double spend in the past. So, yes, go ahead and use ghash. Help them get 51%, assist them in whatever f'd up thing they might get up to and all your precious little coins and mining hardware will be worth absolutely nothing. Great plan! Houseman for president!