It's possible, but not likely that the water is leaking out of the piping inside the wall.
It's more likely that the water is leaking into the wall from the shower head, and dripping off something so that it shows up on the other side of the wall.
Try applying some duct tape or plumber's putty along the top half of any escutcheon plates around your hot and cold knobs as well as along the top half of the tub spout. Water from the shower could be leaking through the holes in the wall made for the faucet handles and spout.
Your tub spout will either screw on or slip on. Look at the bottom of the spout for an opening right near the wall. If you have such an access hole, look for an allen head set screw in that opening. You need to loosen that set screw before pulling the spout off the copper pipe it slips onto.
If the water doesn't seem to be coming through the wall, there are only three places it's likely to be leaking inside the wall:
1. Where the pipe that goes up to the shower elbow connects to the faucet body
2. Where the pipe that goes up to the shower elbow connects to the shower elbow, and
3. Where the shower arm (that the shower head screws onto) connects to the shower elbow.
That is, it's possible you have a leak at the faucet body, or at the shower elbow (which is what the shower arm screws into).
It's extremely unlikely that the copper pipe between the faucet body and shower elbow is leaking.
So, you'd only need to make holes in your daughter's closet drywall at two elevations; one at the elevation of the faucet body, and another at the elevation of the shower elbow.
typically, people won't know what you're talking about if you say "shower elbow". Call it a "winged back elbow" and they should know what you mean because winged back elbows are typically what's used inside the wall to screw the shower arm into.
Your shower arm probably has an escutcheon plate around it. Loosen the set screw on that escutcheon plate and slide it forward. Use a mirror and flashlight to see if the water is leaking out from where the shower arm screws into the shower elbow. Maybe the shower arm just needs to be tightened. If anyone's replaced that shower arm recently, maybe they didn't know enough to put teflon tape or pipe dope on the threads.