It's about the principle of the situation, not the "law". Just because it wasn't written anywhere it doesn't mean that it's not bad. Daniil clearly knows that what he did was wrong as per his reaction. This is ridiculous defending.
It is law, sorry. The moral aspect is simple: its not copyrighted, which means it is free to use. There is no guilt in using something that is free to use.
What aggravates it for me is that she clearly did not try to do any negotiations before starting a public "poostorm", as in contacting the people and requesting to have the copied part changed/removed.
Bully as it is.
Deleting comments is the right attitude: if the author wants some reasonable discussion, this discussion will be done in private. Public discussions are poostorm and bullying pure and simple.
UPD: in fact, she wrote herself that Daniil contacted her to aplogise. That is the right attitute: delete "poostorm" comments, and solve the issues in private.