I don't think it's automatically a problem she comes home. It's better she finds this way of coping with being unhappy socially there, and therefore still gets her degree. Otherwise from what I've seen and experienced of people who are unhappy socially at uni, the other results could be:
She drops out altogether, which would be a shame if she likes the course and is halfway through.
Her mental health could suffer severely
Or she could end up doing things which dint come naturally/ aren't 'her' to try and find friends / fit in, which could work out but equally she may end up feeling even more out of her comfort zone, and in my experience no clubs and societies are a magic bullet and joining late can make fitting in hard.
I went to uni close to home, tried not to go home too often but ended up living at home completely in the third year after feeling very unhappy in my 2nd year house share. I don't regret that. I do kind of wish I'd made a bit more effort to befriend people on my course rather than people who were in halls with me in the first year, but hindsight is a wonderful thing and although I socialised with them in the third year their groups were fairly established by then. Looking back maybe if I'd been further away from home in the first place I would have made more effort with more different friends, but equally perhaps I'd have been equally as unhappy socially in the 2nd year but more trapped and ended up in a worse place mentally or dropping out. I'll never know!
My husband did move away from home, only went back in holidays, but ended up only really finding friends through a church despite never bring religious before or since, so looking back he used that as a 'crutch' in the same way some use drugs or alcohol and he wouldn't say he was that happy, and some of the friends drifted when he realised he didn't really believe after all.
We both seem to be equally capable of living adult life now despite having different experiences of how independent from our families we were at uni! I went away to do my pgce and was much happier as I happened to meet better friends. I then got my own flat as soon as I was working despite it being close enough to live with my parents, so I was quite capable of being independent fairly young despite the year where I'd felt I needed to go home.
So I suppose I'm saying there is no one right way of doing it and sometimes it's a matter of making the best of things for you. Identifying what makes you personally happy and how you can vest manage a situation for you, rather than following what 'most uni students do' is a life skill in itself. Some people on this thread have a very narrow view of how adult life should be / what constitutes success. Plenty of people manage to be perfectly happy always staying close to home! The point of uni to some is to study the degree, not to live some idealised student lifestyle with pressure to make friends for life!
However, I would pick up on your point about not needing to work. While I wouldn't say she necessarily needs a part time job all year, she'd not doing a vocational degree so I think some volunteering, summer work, work experience is important. Does she know what she wants to do afterwards?