Mysterious leaking.

Okay, so this bathroom at my sisters house was remodeled about 6 years ago, just before she bought it. There is water leaking in the first floor bathroom,down into the basement, more than just a drip drip drip kinda thing. But only when the shower is on. No leak when water is going through tub spout. No leak when the toilet is flushed (about a foot away from the tub). I took the shower head off and tried the shower again and no leak. So I’m thinking the back pressure of having the shower head is causing it. Now, from down in the basement, I can see the water is dripping from a nail that is between two floor boards, and is halfway between the toilet flange and where I see the tub drain. The water is obviously coming from a water line, but my question is this: what do I tear up first? Tearing up the vinyl floor in the bathroom I feel is useless because that’s just gonna show me a wet subfloor, right? The vinyl floor in that area is spongy and starting to lift. It’s more than 16” from the vertical copper shower pipe, so the water would have to be going through a stud to get to the exit point, right?

I’m a bit stumped….

Thanks in advance!