sounds more like your mum cant ride to begin with (no offence intended)
you have to remember and as silly as it may sound the purpose of a cbt isnt to teach you how to ride, the instructors have no obligation in doing so. The purpose is to educate you on how to ride safely, teach you the main things of the highway code, teach you how to use your bike i,e maneuvering it whilst on and off the bike and about safety checks again on and off the bike i,e safety checks of the bike and safety checks whilst riding the bike etc then finally you have roughly an hour and half on the road where they look to see if youve taken all of that onboard, hence it being very hard to `fail` or obviously very easy to fail if you cant ride to begin with.