Yes, there is a way. You left them specifically opt in to it, you provide them disclosure documents of what's being tested, and you provide free access to mental health resources.
It would be interesting to find out if these experiments lead to any suicides etc. Although I suppose that Facebook is not interested in releasing that type of information.
Which drop of rain caused the dam to break? In the book Good Omens there is a demon who considered himself the best not because he was the best deceiver or torturer, but because of his ability to cause mass malcontent.
The real evil isn’t in the experiment, it’s that the researchers found that Facebook makes people’s lives worse. How many suicides are caused by Facebook’s investors? Whether they can mitigate it this way or that is splitting hairs.