You could try getting an agent, but to be honest she is likely to find it tough unless she's going for extra work (which could provide interesting/useful experience and some extra income, providing she can find flexible part-time work as well).
The competition for agency representation for young women is very, very fierce. If she looks really young for her years or has unusual looks or skills she may find someone to take her on their books. Agents are inundated for requests for representation. Many drama school graduates don't get representation straight away - and that's with the benefit of 3 years' training, drama school productions and a showcase and all the industry contacts the schools have.
I have two grown-up children: one is an actor, the other is currently training at RWCMD. My son didn't have a gap year, but my daughter did: she spent her summer doing stage-management and then the rest of the year travelling and earning money. She was fortunate though that she knew she had a place by Christmas (this was for music, not acting) so it did free her up to travel. The trouble with drama school applications is that it takes the whole blooming year, so it can be difficult to plan anything that takes you out of the country.
My top recommendation for a gap year is to fit in as much theatre as you can (either as participant or audience member) and to read as many plays as you can. Looking for really good audition speeches is a long job!
Any theatre she can experience as an audience member can be useful: local amdram, fringe festivals, local Rep, West End - anything and everything. Trying to work out what "works" and what doesn't can really help in the preparation for auditions. Sometimes you can learn more from watching less-than-brilliant acting than you can from watching the best in the business - providing you watch with a critical eye.
Foundation courses might be useful, but they're not essential. I think RADA's is expensive because they know they can get away with it! Here's the comment from NCDT:
http://ncdt.co.uk/guidetotraining/courses/coursesfaqs/
Are foundation courses necessary and are they worth the cost? How else can I prepare before going to drama school?
Most accredited institutions now offer foundation courses, for those who wish to prepare for two or three-year programmes. Currently, the National Council for Drama Training does not accredit any foundation courses. They are by no means necessary preparation for drama school, but can be useful for some students. It’s a question of what’s right for you. The fees can cost more than £10,000 for one year, and there are no substantial sources of funding available.
There are other less expensive ways to prepare for drama school and build up performance or technical experience. Get involved with youth or amateur theatre in your local area to learn skills, get used to working as a company, and start finding out about yourself as a performer or technician. Make the most of the facilities that schools provide, set up your own drama group if there are none nearby - all of this will strengthen your application. Consider a FE course or part-time programme.
Drama schools also value a breadth of experience that is not confined to performing, so don’t worry if your CV isn’t filled exclusively with acting or stage managing.