Not mentioned here, but the first bike shop tried to put your 5'10" frame on a fracking 24" bike??? I am 6' with extra long inseam/arms and I fall between a 19.5" and a 21". Run your measurements on the fit calculator at competitivecyclist.com to get an idea of what size you should be looking at. Then try the sizes to see what actually works for you.
Dont know much about Devinci. Given a choice, I'd go for a proven frame from a brand like Specialized, Cannondale, Trek or Giant. The Specialized Rockhopper may be a bit above your budget but will probably be one of the best all-around options in this range.
The Boulder should be fine for your fiance.
V.