Funny thing on the 29'er aspect of this discussion. Though I'm a long time MTB'er, I only recently made the jump to a 29'er....Trek Rail 7. At the bike shop we experimented with 29/26 and 27.5/26 combos, but I never owned a full 29'er. On the turning capability of a full 29'er, this was probably one of the biggest surprises to me. Maybe because I come from a dirt motorcycle background, but even compared to my 26'ers, I don't find the 29'er to be the trying-to-herd-elephants handling that I expected or hear about.
These big hit, long suspension 29'ers have huge tires with lots of traction. Their ability to be thrown into a corner and leaned over at will is tremendously increased. The "leaned over" part is the element I'm especially focused on. And hey...I'm not trying to defy physics, bike geometry, etc., but "leaning over" is how you partially steer a dirt motor through a corner...and of course counter steering, body placement, etc. The same applies to the emtb IMO, and I kind of think that maybe quite a few mtb/emtb riders are cautious about aggressively leaning their bikes. I'm not saying to diss' a lot of bicycle riders.
Dirt motors are bigger, heavier, and faster overall, so you really have to build confidence and technique for aggressive cornering. I am not claiming that dirt motors and emtb's are exact clones in the handling department or the applied techniques, but they have way more in common than not. I still contend that the OP's lady is more likely having issue with technique due to the more aggressive nature of an emtb than she had/has with her pedal-only bike. And not saying any of that in a disrespectful manner. We can all have more to learn about improving our riding skills when things change.