Definitely get a FS bike. Either a 27.5 or you can probably get good deals on second hand 26" bikes if you're not sure if she's going to love it. Something with 140-150mm travel that pedals and descends well.
Lessons are a great way to learn. I wish I'd done some sooner. Endless Biking has good lessons and remember their motto "Saving marriages one lesson at a time".
That being said go riding as a couple and encourage her to ride. It's good to do the activity together. Accept the fact you'll say something stupid and make her angry at you.
NSRide is another riding club to consider. It has beginner and girls ridges on Tuesday's a co-ed on Thursdays.