I believe typically it's required to have completed high school studies (HS diploma or GED) and in any event UK universities will more or less universally require US students to have done APs or IB or another A-level equivalent.
The SAT is not equivalent to (or replacement for) either GCSEs or A-levels. It's a university admissions test, effectively. Not all UK universities require it from US students, so she would need to investigate ona case by case basis. In terms of level of content it's roughly GCSE level material although the question format is very different and a lot of success on the SAT circles around just knowing how to do the test and approach the questions (i.e. "exam technique").
As far as I am aware the SAT can only be taken at an official test centre, and it consists of two sections, taking about 3-4 hours I believe (the old one I did had 3 sections and took longer).
Realistically she should look at either a) just doing A-levels by distance learning or otherwise or b) getting her GED and doing APs or c) going to community college in the US to get her AA and then applying in the UK (she'd probably still be applying for first year entry in this case though).
No matter what, it's not possible to go to university in the UK after having left school before the equivalent of GCSE study and not having completed any further study since then. Even if somehow a university did accept her (a few less scrupulous ones maybe...), she would be wholly unprepared for the course and likely struggle immensely and quite possibly drop out (or fail out). Which doesn't really help her at all.