FWIW, I am likely going to throw together some parts on my old Traitor Wander that I used as a testing ground for road disc (spoiler, built a Lynskey disc, I'm sold) and use it as a flat bar, road geared singlespeed. For width, I am going to start with a goofy set of Niner... crap, what are they called... they were a bizarre, angled bar that I run on my MTB with some Ergon's I have in the bin. Definitely wide, but I don't really care, and I already have them. I would honestly just find a cheap bar, see if she likes the width or wants more or less, and go from there. Since you have already been VERY well informed that you shouldn't do this for whatever reason, and honestly may be giving the bike slightly different than intended geometry, just try something and see how it goes.