Both your statement and your parent's are probably true. Extended forced isolation is incredibly psychologically damaging. Likely more so than being forced to interact only with "hardened criminals".
I'm confused. "It's not true" -- you seem to be saying that solitary confinement is better/safer than normal prison life. But then you say that people in prison are not so bad.
I don't think that people in prison are "inhuman", but I do think that they are "hard" in a way that would be extremely difficult for me as a very sensitive person. I think they'd have to be to get by in an environment like that. For example I can't watch Orange is the New Black -- it stresses me out.
Solitary confinement is not 'not having a cellmate'. Solitary confinement is being locked in a tiny concrete cell, with no bars, no windows, and no chance to speak with another human being. Indefinitely.