Yes you can call the person an attacker. The rules of a mature Ethereum protocol should be neutral to the intentions of users, including those that one would reasonably characterize as hackers, but Ethereum was not a mature protocol at the time. It was effectively in early-stage beta. The DAO was the first smart contract of its kind, and was expecting $500,000 worth of ETH to be deposited in it. Instead $150 million worth was deposited.

I believe that a fork like the DAO rescue would be perceived as totally unjustified and impractical today, and Ethereum is still a very young and experimental project. I think in a few years, when the network and technology are mature, such an application-rescue HF would be unthinkable.

Regardless of any of this, the person didn't attack anything.

I'm not saying my position on whether the fork was right or wrong, but I think folks on either side should not characterize that incident as an attack.