I read the question 3 times and suggest your friends daughter concentrates on learning about law, rather than sounding like a lawyer
Is she asking how are the inexperienced restricted from supervising and how are the experienced tested for experience?
This thread answers the first part of that.
In reality you could pass your test at 17, never drive a car for the next 20 years and then supervise a learner. However, you might struggle to get insurance to do so.
I remember passing my car test and bike tests at a young age and thinking I was an amazing driver - I went on to do various advanced tests and instructor of advanced test and realised I wasn't very good at it at all .
The real issue - is that you have to have sufficient experience that the process is embedded in your brain such that you can do it with spare capacity to supervise others. For example:
Riding a motorcycle to an advanced test standard requires a fair bit of practice.
Doing so while giving a commentary over a radio requires a lot more.
doing so while giving instructions over a radio and observing the acts of two other riders a bit more than that..
It really has to sink in before you can go teaching others
In contrast - you could be a yacht master at 18 in charge of a £250,000 15 tonne boat, or an airline pilot at 19.