Probably simplest and easiest route would be Claife Hieghts imo.
Presume you have a map? Mate has mine so from memory;
From Ambleside head out towards Hawkshead the take the road down to the Lake side at High/Low Wray. Cycle along Lake side on BW. There is a steep cobbled track up to your right, take this up onto Claife Hieghts. Then it`s a forest road for a bit before descending the BW, basically through a farmers field. You will come out at Far Sawrey. From there you can pedal back towards Hawshead and then Ambleside if she has had enough or turn left and take the BW back over to the Lake and along the same Lakeside track back to High Wray/Ambleside.
Short, simple, with some easy escape routes and the all important cafes en-route at Hawkshead.
If you do go to Biketreks go early or day before, busy people. They also have printed maps of the good bits in the area.
Nipping down to Grizdale may be a better option for GF? Trail centre with route markers for easy navigation