Tough to provide proper guidance through the 'puter, just because all the magical tips and general guidance out there are rarely applicable to everyone. A good tip or swing thought for one player might be counterproductive for another. I'll offer a couple of ideas that will hopefully help with this.
First off, when making changes, do not get hung up on where the ball is landing right away. Changing swing timing, geometry, etc. means un-learning old habits and the first priority is to pay attention to making solid, consistent contact in the wake of some alteration. Once you establish that contact with the ball, then you can eventually begin to narrow down shot placement.
It sounds like there's too much arming of the ball after an okay setup. I like to promote the thought of a step-and-a-turn for a good move through contact. You could have your wife hold a ball in her racquet hand, take that forehand set position, and use that step-and-turn move with a loose arm to throw the ball and get more used to that feel of driving the move with her lower half.
Usually that grip change requires a leap of faith where a player needs to essentially change to that grip and make a mess of more than a few shots. I like to reassure students when they "feed the net" with a more closed racquet face so that they understand the change that's happening - those first shots should go down into the net. Typically the next step is to encourage them to begin lifting through contact to make that topspin. That need to lift through contact should get her to want to come up from under the ball.
If she's using a sort of "alligator arm" to swing at some forehands, I try to generally encourage players with that issue to simply get further away from the ball. I might also tell a righty to look at the ball like a clock face and try to hit around the outside of the ball at 3 o'clock (from the hitter's perspective) instead of right through the back of it. That thought can bring much better extension to a developing stroke in a big hurry.
Along with that improved extension, you want to encourage a swing through the ball instead of just to the ball. I love the cue of following through on the forehand side so that the elbow points in the general direction of where the hitter is trying to send the ball. Lots of good things happen with that move in terms of racquet acceleration (instead of deceleration) along with promoting a better shoulder turn through the stroke.
Consistent feeds help with developing these new habits and pace of these feeds isn't important right away, so I like to deliver hand feeds from the hitter's side of the net. Since I'm closer to the hitter, it's easier to see what's happening and offer cues when making fundamental changes. After the shot starts to fly a little better, I'll retreat to the other side of the net and feed more from over there.
Remember to limit the alterations so that you're focusing on one change at a time instead of five or six little things all at once. That should keep the potential frustration under control. Expect to be spraying the ball around at first - that's a sure sign that the forehand construction zone is open for business.