I think the situation described to me for people with provisional only was that the provisional bike licence was valid for 3 years and the CBT for two, but (this was around 2000/2001) you had to wait a year after the 2 years had expired before redoing CBT.
That sounds about right actually. (I passed my bike test in 2000 also).
I’m surprised if retaking the CBT is a much cheaper option for someone who has been riding on the roads for two years so presumably fairly confident.
She’s confident and a good rider for her experience levels, but hasn’t had any lessons beyond what pearls of wisdom I’ve been able to impart. Whether she can ride in a manner appropriate to pass a test or not though, that’s a different question.
I’d have thought she’d need a couple of lessons to iron out any bad habits, at least. But you’re right, the cost of the tests aren’t vastly greater than the cost of CBT. I’d guess maybe time might be a factor too, if she’s left it too late to get around to it (she was suffering from a long-term illness which hasn’t helped) and her CBT is about to expire.