A student submitted a paper in an art history class that contained some ideas from art critics she read about, and whose ideas she agreed with. The student didn't cite the critics as source, but claimed it wasn't plagiarism because their ideas were merely their own subjective judgments, or opinions, not facts or findings; furthermore, they were opinions she agreed with.
Yes, it's plagiarism if she learned novel or interesting ideas from an art critic and then used them in her own paper without crediting the source. It doesn't matter whether the ideas are facts or opinions.
For commonplace opinions with nothing novel or interesting about them, there's no need for a citation. However, that's presumably not the case here, since one couldn't attribute such opinions to a specific critic.
the practice of taking someone else's work or ideas and passing them off as one's own.
So, in this case, the source (art critic) or destination (art history class paper) are irrelevant; it is still plagiarism.
It might seem unfair that ideas get accredited to the first author, but that is the game. Publish (early and often) or perish.
Technically is it not plagiarism if someone honestly arrives at an existing idea independently (reinvents the wheel). Not the case here, and impossible to prove anyway. That would be considered negligence, since a disciplined researcher would find any existing references.