I'd really listen to her saying she wants to be social work, yes it is a tough career and social workers are often critisised but if it is what she is interested in, it is clearly something she has put thought into and she could have a very long, successful and worthwhile career.
Focus on this, re,ind her of the grades she needs. Maybe there are some related careers that she could look into e.g. working for a charity like NSPCC.
Put an action plan in place with her to achieve this goal (even if it isn't what she ends up going into, what she needs is motivation for now). Sit down and be positive and supportive of her ambition. be glad she has this ambition and it is a respectable and not an 'easy option' in any way.
Come up with a list of courses and what she needs to achieve.
A plan for if she fails (repeat the year, different courses, a college vocational course in something related)
A plan for her to get work experience relevant for her application (this will show her you take her choice seriously and give her the confidence to believe she can achieve it, hopefully making her work harder)
A plan for THIS holidays how she can maximise the time, what will motivate her to revise, how you can help her revise, I think at this stage she needs to be learning material and looking at past papers and mark schemes to understand what the examiner is looking for. Reassure her it isn't too late (it possibly is, but telling her is just going to demotivate her further) and that if she really wants the Bs you are happy to do whatever to help her and that you believe she can achieve.
Moaning at her and getting cross is just going to piss her off and make her put up more of a stand. You've tried the strict tactic so now try the encouraging and motivating tactic!
Good Luck