Hi and welcome to you and your wife.
I have a navigator 2.0 and this summer I got a road bike. I did it because while I love the comfort of my navigator I had a very hard time making it up steep-ish hills.
My husband and I both use our mountain-ish bikes (I have the navigator, he has a specialized hard rock) when we go on trails. The trails are rails-to-trails so the hills are at a very low grade. I love my navigator on these trails. Especially because I'm usually attached with a bike trailer for my daughter to push behind me.
I now have a road bike and it really is a great bike for getting the increased speed and ease up hills. I'm not sure what the best bike would be for her but maybe someone else will have a suggestion. I wonder just how much better a trek 7.3fx would be over the navigator. I don't think they are really that lightweight.
I would suggest you take her to a few bike shops and put her on a cyclo cross, touring bike, even a road bike.
When I bought my road bike this summer, I got a used one for a lower price. It was a good choice b/c I also invested in clipless pedals and ended up falling over about 5 times. I'm glad I didn't fall over using a brand new 2,000.00 bike