There are some really great suggestions posted here.
You mentioned your wife 'hates off the saddle pedaling'. MTBing is about 'body english'. It's not like road riding where you spend almost the whole time in the saddle. Guiding a MTB down a trail requires subtle and not so subtle body movements, be it arms, legs, hips, shoulders, body weight in front of or behind the saddle, to the left or right of the saddle. These movements are usually accomplished standing on the pedals (usually with knees bent to some degree), or with your backside just grazing the top of the saddle, without your full weight on it.
Here are a couple of tips for when she's back on the bike. These can be done in a parking lot or a grassy field:
Most of all practice riding out of the saddle.
Get a section of 2x4, or 4x4 and practice riding over it standing.
For balance, ride the bike in slow circles, to the left and then to the right. *Make the circle get smaller and smaller. Sit in the saddle keeping the bike vertical to the ground, and lean your shoulders a bit to the side you're turning to keep the bike from falling over. You may have to stick out your knee towards the inner circle.
Ride figure 8's.
Ride on the white lines in a parking lot trying to stay completely within the line - sitting and standing, fast and slow.
Have a mini "race" against each other, but see who can get from point A to point B the SLOWEST without putting a foot down.
Practice riding up and down on curbs.
These skills techniques can be used as a bit of a warm-up before getting out on the trails.
Oh, and always look where you want to go, don't stare at the front wheel or the ground immediately in front of it.
You're in the right place on mtbnj to find other riders willing to help out.
Good Luck!🙂