Which document will she take to the test? BRP or Passport? Does one of those have her married name on it?

Whatever the case, I believe when you then submit future ILR etc. visa applications you will have all the evidence to show her maiden and married surnames and the person processing the documents will see and understand this, it won't be an issue. In my case, when I took the IELTS I used my maiden name, but now my BRP/passport and other documents are in my married name. When I submit my ILR application later this year I will have a similar 'mix' of surnames.

I'm sure someone else will be along to clarify, but this is my understanding.