My DD is coming out of a slump that has been about a year or more. She went in 10u hitting pretty good for doubles and some triples, home run, etc. Then in 12u she completely flattened out. Pop ups, late, fouls, strike outs, watching pitches...you name it! Throughout all this, I never ever asked her to "choke up", "just put it in play"...to me, that's a defeatist attitude to take whatever you can...because you basically have to because you're skill level sucks, your attitude sucks, whatever.
Hitters should swing for the fence especially when they're down in a slump. They should become extremely aggressive - yet focused. So far, my DD has been climbing out with an in-park HR couple weeks ago, before that double and RBI for 2 runs...she's coming back. Without her keeping on ripping each and every swing I have no doubt she wouldn't tempered down her desire as well. Tell the body to slow down, over time in a prolonged slump I believe the mind and motivation will follow too.
I'd tell her to keep up the hard work, keep swinging for the fences, remain focused and keep working the mechanics at FULL THROTTLE. Her next mis-hit could turn into a triple or something productive.
Get a video, you must. Chances are you just need to do much more front-toss so she can get timing back. Ask your DD how she looks at the ball, where she's looking at the ball as it's coming. Video will tell you where she's hitting the ball or missing - too far out front, too deep. That will let you know her timing is early or late.
Is she popping up or grounding out? Try adjusting her vision, ask her to look at the top half of the ball. Believe it or not, nearly all batters have no idea what they're looking at. It took me a few weeks to get my DD to learn where she's looking at, which was at the bottom of the ball as it's dropping on the way to her. This causes under-swings resulting in misses under or pop ups. The ball is dropping naturally and hitters over compensate that drop thru vision.
DD now looks (or tries to) at top half of the ball and to "swing up" at that half. Gravity naturally drops the ball so in effect you have a better chance of contacting the middle-ish area of the ball. A mis-hit will result in a hard-hit grounder but you'll see more contact possibly if it's her vision. Try this, you just may see a difference.
If she's only grounding out - reverse it. Have her try to look more at the bottom half of the ball and to again, swing up at it.
When I say, Swing Up - that's just a cue to keep the swing from flattening out in a desperate attempt to "make contact". That's all, nothing more and it does take some explaining and reminding them. At 12 this shouldn't be confusing after a couple weeks of practicing with this cue.
In fact, video can show you if she's even looking at the ball when she's swinging. So much video can reveal, then you or coach can put in the reps.