My first bike was a 72 CB750 and I did alright on it. I will say the first few hundred miles were a steep learning curve. I'm a tall guy so can touch the ground both feet no problem but it is a heavy bike to learn on. I would have been safer and learned faster on a lighter bike. Shortly after getting my first CB750 I got into dirt bikes and had an XR200, perfect to learn on and practice technique. Later got a CR250 Elsinore which was my favorite dirt bike... Felt very matched to my abilities though was a bit tall even for me.Later added an IT465 to the stable, which was a wicked fast bike but I never enjoyed it as much as my Elsinore or even the XR. Never could ride it like it should be ridden, too heavy and twitchy. Fun in the open sand pits and hill climbs, but was afraid it would kill me on trails and I couldn't run laps as fast on that as the Elsinore. Riding dirt bikes really helped me on the street. Starting with a smaller (lighter) bike would be an advantage for her IMO.
For comparison my girl is intimidated by my CB750. She is leaned over a bit to touch the ground, and the one time she rode it she was afraid of the power. FWIW she rides a VTX1300 so is used to a heavy bike. She wants me to build her a CB, but she wants it lower. I had been suggesting a 500 or 550 which she always said she doesn't want a tiny bike, but after riding my 750 she has been receptive to a smaller bike. She's hung up on numbers and displacement, used to the massive 1300cc engine of her cruiser but the thing is slow and handles like a yacht. Not sure what we'll end up building for her but it may well be a 550 instead of a 750, though a 750 would be easier on me from a parts on hand experience standpoint.