Work experience/extra curricular activities are pretty much vital due to the course being so competitive to get a place on, so i'd advise that she starts thinking about things now that she may wish to proceed with once the covid situation allows.
At the moment she may be too young to be accepted as a volunteer for many things or to get paid work experience, but she may be able to participate in things like the Duke of Edinburgh awards soon.
If she can show initiative in other ways it would be beneficial, especially if it's doing something which shows her ability to work well with people and manage responsibility, team work etc. She may want to try and arrange something for charity, organise shoebox donations, or try to organise something through school such as mentoring younger students.
Hobbies with transferrable skills can also be beneficial.
This would be in addition to making sure she is taking the "correct" subjects for her GCSE and A-Level options of course.
Eventbrite is a great website for searching for online seminars, webinars and training sessions for various things. I participated in several to help strengthen my NHS application for example and was offered an unconditional place.
www.eventbrite.co.uk/
There are many such as "So you want to be a Doctor" and "Get into Medicine" which may be beneficial.
Many free options, some have a fee. But do search and see if there's anything that takes her interest. Often you'll get participation certificates, which will help strengthen her application.
Good luck!