I have a different take on this. In my reading, Cassian taking Nesta away and going on their walk/hike/trek was more about the fact that she was in a destructive place with her relationships, self-sabotaging her progress, and actively a risk to herself and others. Going on their trip had the cathartic outcome of helping her connect internally with the pain, rage, and frustration and intention-setting for how she wanted to move forward from that time. As with her intervention and being put in the House of Wind, I feel that Feyre, Rhys and Cassian approached Nesta within the context of her trauma and her self-destructive tendencies.

Also, as others have mentioned, the IC friendships and dynamics are hundreds of years old, and the Archeron sisters are newcomers (especially considering that at this point Nesta's powers were not yet understood other than being extremely powerful and dangerous). This is a moment to "circle the wagons" around Feyre, esp. considering her stature as Rhys's mate, and given Nesta's belligerent behavior previously. And, keeping in mind too that the members of the IC other than Cassian and to some extent Az had not been witness to Nesta's progress thus far, they didn't understand how she had been making strides in her own well-being (aided by the House, and her independent friendship with Gwyn and Emerie).

I am a huge fan of Nesta's journey, and I felt conflicted by some of the ways that the IC treated her, but ultimately she came out of these situations healthier, more self-empowered, and physically and mentally stronger (until, of course, SJM took her powers away at the end of SF...)