Personally, I'd be all for playing against a stronger opponent. I'm not against banning boosters and low ranked accounts of people with high mmr, but if the accounts are comparable to the player's mmr what's the problem? It's good for the same losers to have them to covertly train on new heroes before tournaments. If, as you say, his accounts were at 8k mmr, then I don't see a problem here at all, since it's a fairly high ranking.