I can give you definitive information on this (I know the statutes well and regularly handle subject changes). It is only possible to combine English and Law if she is willing to do a four-year degree. English is one of the few Triposes which still have a two-year Part I (i.e. there are two years before the students take their first University honours examination). This means that she would have to do two years of English before being able to switch to Law. To fulfil the professional requirements to practise in law she would then need to take the full two years of the Law Part I (IA and IB university honours examinations).
The other way around also needs four years: she would need to complete both parts of the Law Part I before changing to English, and students who don't have Part I English are required to complete a two-year Part II in English. So it would be possible, but it would take four years for either route (with the additional costs of fees and maintenance for an extra year). Set against the cost of a Diploma in law year for non-law graduates, though, and the fourth year may look less expensive (but you would need to check that all of the professional exemptions could be achieved in just the Law Part I).
Sadly there is not any way of getting around the need for the extra year, as this is required by the way the University's statutes work (e.g. how many qualifying honours examinations must be taken and when in order to graduate). I have known a few students switch between these Triposes though, so it's not uncommon.
Of course, it also depends on whether the fellows in the college will allow a switch. If done part way through the course, most colleges have an unofficial policy of only allowing subject changes if a student achieves a 2:1 in the original subject before being allowed to switch. It's at the discretion of the fellows in the destination subject whether to allow a change, however; and they will normally require the student to take a full admissions test and interviews before deciding (and the student won't be accepted if s/he does not come up to standard compared to the usual admissions criteria in the subject). If a student is applying to do a split Tripos at the outset, s/he will normally be required to satisfy the fellows' admissions criteria in both subjects before admission. (This isn't easy - bear in mind that the admissions criteria - not just the grades, but the style of thinking they are looking for - in different subjects can be very different, and students may be suited to one discipline, but not really to another. Admissions staff and academics are not really looking for all-rounders, but for students who have an ability, potential or feel for that specific discipline. It's actually very unusual to see a candidate who would be equally suited to more than one subject). In the case of Law and English, both are very popular/competitive subjects in admissions terms - English possibly harder to get in for than Law, even!
If she enjoys English more but wants to be a lawyer, it's very common to take the professional conversion year/diploma year in law after an English degree, so it's very possible to do it by that route.
I hope this helps - do PM me if you need any further info (or contact the Admissions Tutors direct at the colleges your DD is interested in - they will be happy to help).