I did my CBT then bought a road legal dirt bike and rode that for a few months before having a few incidents that stopped me riding for a year or so.
Eventually had to redo my CBT (they expire after 2 years) did three lessons on a big bike and took my test. Didn't ride for 5 months then bought a CBR6 then went straight onto 748r.
As has been said it really does depend on confidence, general driving ability and natural ability.
ie, if she is confident in a car, has good awareness, has ridden pushbikes and is confident in general then she could effectively ride any bike straight away.
Given an option I would suggest to anyone that they ride a 125 on the road for a while, at least to get the experience of how little car drivers notice you and get used to handling a bike in general. But I would then go for direct access test so she is not limited to what she rides when she is ready.
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