Like I said, findign a job is paramount to her becoming more responsible and independent and also to your parents viewing her as an adult and therefore respecting the decisions she makes. This needs to be made clear to her during that discussion. This guy is only a symptom of her carefree attitude towards life and I'm sure once she becomes her own person she won't want anything to do with him after a while. The more confident she becomes, the more she will want to have someone by her side who deserves her.Right now she might be settling for soemthing less because she doesn't feel confident enough in her own skin to aim for something better.This is why they should let her date the guy if she wants to. This way they are telling her: date whoever you want but know that as long as you live in this house (and therefore you are not independent) there will be rules that you will need to abide to and for instance, you can only see him x times of the week or something like that.This way becoming independent will be like a carrot to her; something to aspire to in order to be able to live life on her own terms.
He is not the problem. She is. She needs to realise she has to take responsibility for herself and everything else will hopefully fall into place when the time is right. Your parents need to be very firm about the timeline she needs to complete her degree and get a job in and make it clear that after that they will not be willing to support her anymore.I know it sounds harsh but sometimes tough love is necessary to help our loved ones. She has plenty of time to find a job; besides she can start doing a part-time unpaid internhsip while she is still studying to boost her CV so that she will have more chances of finding a job once she completes her degree. ALso, she needs to realise that she won't find the job she wants immediately after graduating so she must be willing to compromise and build up her experience to get her dream job a few years down the line.
Open commiunication, love and understanding on both sides is really important for this to work.Your role could be that of an impartial third party who is willing to offer advice and support to both your parents and sister during this time.Don't make either of them feel like they can't talk to you by appearing too distant or unintrested in what is going on but also you should retain a reasonable , impartial view on things to be able to offer constructive advice and so that both your sister and parents don't feel alienated.