It sounds as if she does not feel completely in control of the bike; I remember this feeling, it’s horrible! Riding in non-consequential terrain is the place to start.
Remember that what will seem like bland riding to you will be demanding and unpredictable for her.
On a grassy slope you could practise braking when descending out of the saddle (aiming for a short stop and no skid, weight back), basic cornering technique (outside foot down, looking through, no front braking and feeling how bike lean affects the turn) and riding diagonally across the slope to feel off camber control both up and down hill. This will develop her ‘cone of movement’ and bike handling skills.
The above are simplified coaching exercises that work. She needs to be in control of the learning and if you give her feedback it needs to be linked to what she did right or needs to improve: BE SENSITIVE! Praise the effort and do not give too much corrective feedback.
The confidence on the road will come with time, once she feels in control of the bike. Maybe ride more with her in traffic, it may be worse if you shoot off ahead and she has to catch up and deal with cars. I always ride with my Mrs in front 😉 she sets the pace and doesn’t feel rushed.
This is going to feel like a slow process to start with, finding something difficult when you’re with an experienced person can make it feel more frustrating.