I would strongly suggest the Bac ES.
Unis in the UK do not require the Bac S for law applicants, and the maths will bring her marks right down.
The maths in Bac ES is much easier and much more relateable for students -- they can see the point of it. So my son, who was struggling in maths through Seconde, rose to close to the top of his class after going the ES route... and scored 19/20 in his final Bac for maths. I reiterate, he is no maths genius and would have scored poorly in maths had he gone the S route, which as you know would have had dire consequences for his moyenne générale.
Bubbles, the French Bac is marked out of 20. You're citing IB scores there. For a very good law school in the UK, a prediction of 16 to 17 would be good. Here are some data from this cycle just gone, re admissions criteria for French Bac students:
UCL Law with French Law -- 15.75 MG with key subject scores of 17, 15 and 15 plus a good LNAT
King's London English Law and French Law -- 16 MG plus LNAT
Oxford Law with Law Studies in Europe -- 16 MG plus an excellent LNAT score (30 minimum in the multiple choice section)
LSE Law -- 17 MG but no LNAT
Warwick Law with French Law -- 16 MG, no LNAT
Bristol Law with Study in Cont Eur -- 16-17 MG plus LNAT
Kent English Law and French Law -- 14 MG no LNAT
Exeter English Law and French Law -- 14 MG no LNAT
Nottingham Law with European Law -- 14 MG plus LNAT
Note the competitiveness of some of these courses -- as high as 15 applications per place on one of them. This means that a prediction just a little higher than their standard offers will help impress (but above all, a very strong LNAT will be what gets you past the first "tri").