a 125cc bike is rather underpowered for riding 2-up. not sure how it is in the uk, but in the states, to legally ride 2-up, u need the full endorsement. if the cbt is on par with our learning permit, i wouldnt ride a passenger just yet.
u’d have much better results if u are able to swing a bigger bike. if she tops out at 115 pounds, u’d be much better off runnin a 883 (or equivalent). be advised that having a passenger turns it into an entirely different animal, especially in the braking area. the slightest weight shift on her part, and u will notice it big time. until u get very comfortable with the bike in the twisties, i would suggest u ride solo until then. 1 wrong move with a passenger, u will go down very hard.