I’m 5’6″ (5’7″ on a good day) in my socks, and ride a 16″ Scandal with a 50mm stem and no spacers underneath on a 100mm Reba.
I can confirm it is awesome, and I’ve sold my lovely 26″ singlespeed and FS bikes, as I didn’t ride them any more. Next bike likely to be a 29er FS.
I have tried 650b (a superbly pimped out Bronson last summer) but it felt a bit dead and like hard work compared to both the 29er and the 26″ FS (an Orange Five).
I ride in the Surrey Hills, btw, and it’s pretty smoof over here.
HTH. Happy to give further advice / encouragement if needed. If Mrs B2B already has good bike handling skills, then this will make her much fasterer. I’ve been riding about 20 years now and tend to be respectably quick on most XCish stuff, and it certainly, according to Strava at least, helped my riding.