I am the female 1/2 of our team and I've been driving our 38' motorhome since day one 11 years ago.
I fully agree that she should be able to drive in case of an emergency or she could get stranded or put herself in a dangerous situation.
All that aside...she needs to be comfortable driving. I'd have her sign up for an RV driving school and then let her practice on quiet roads, then even in more traffic or tricky situations.
It does not pay to only know how to drive in ideal conditions because it may be she'll take the wheel on a quiet road then drive into a construction zone or end up on mountain roads, or heavy city traffic or a sudden storm. My second time behind the wheel I ended up in a long construction area with narrowed roads with barriers...not fun but I had the skills and confidence I could handle it.
Every traveling pair needs a back-up plan in case the main driver becomes ill or worse yet dies suddenly.
Encourge her without forcing her. Get her on this site to read posts and ask questions.