She has two good offers, so fair play to her.
Grant Thornton is an excellent firm. She is likely to get a better and more interesting mix of experience of small, medium and large firms, whereas with Deloittes, there is a risk that she might only work on large multinational audit clients.
However, Deloitte is one of the Big 4 firms and as such it carries a prestige which will stand to her for the rest of her career. It puts her in the top division. (The Big 4 probably accounts for around 50% of all trainees, so it's not that exclusive.)
Many people who train outside the Big 4 feel that they need to do two years' post qualification experience in the Big 4 to round off their experience.
Note: I trained with Deloitte Haskins + Sells at a time when it was smaller than Grant Thornton is now. So I would have got the Grant Thornton experience with the prestige of a Top 4 training.