I think we can't really know what went on with her parents and her family. And just because they are middle class/respectable doesn't mean things were good. My mum, who I suspect is a narc, makes me very wary of accepting help from my parents because it comes with strings. I will do everything I can to avoid ever being in the circumstances where I have to stay in my parents house again. If poverty was more attractive to Jack than staying with her parents, I suspect its because there is more going on there than we can know.
Regarding short term poverty. I had a few years where I was struggling to make my career. I'd used my savings on a masters degree to get me into a specific industry and the next four years was going from one short term, poorly paid job to another. Each job was in another part of the country and my finances slowly dwindled. I ended up with depression and anxiety as I felt myself a failure, I berated myself for not being good enough to get decent jobs, and I had no support network to fall back on because of the moving (and above family problems).
Eventually I found another career route and I got out of that industry and I'm now in a better situation. However, I get rather terrified of being in that situation again - so I can understand the fear and trauma that even a short time on benefits can induce. However, I wouldn't call myself 'working class' due to four years of (and in one case less than) minimum wage work. I also wouldn't count myself as poor because I can only afford to live in shared houses. I also wouldn't think it my place to advise people who live their entire lives in that situation because I had (comparatively) 5 minutes of struggle. In that respect, Jack can come off as a middle-class saviour.