The MTB that I do one could do 95% of it on a gravel or even cheapo steel-o city bike. There are some more technical and rocky sections where I ride but it involves mostly powerline access roads, doubletracks and fairly easy trails. The most technical element is significant elevation change. I am pretty fit and have started to bike centuries and while those did make me very tired I have no qualms about pedaling up 15, 18% grades for a few miles and have done so without issue so I think I can provide plenty of the power if she only contributes enough to overwhelm the bigger bike's weight and hers that's just fine. I own mountain bike only because it makes it a lot easier, smoother, nicer and allows you to talk with a friend or two instead of concentrating 99% on the trail and feeling like you're going to get knocked off your bike but it can be done with "lesser" bikes though not fun. This is not hard stuff I have in mind for the most part.