I only glanced through, but just have the kid/parents buy the music. I'm not quite following if you were loaning out your book or what at the beginning. If they're playing/practicing the piece, they should own it (or at least use the original copy).
If you want to get technical for copyright, you already infringed by giving them the lyrics. Lyrics are pretty much the same standards as the printed music from what I found out. (Yep, everywhere the lyrics are printed it breaking copyright.) There's still the lyricist who created the words. You own a copy of those words. You create a copy and gave it to someone else and prevented the sale of those words. Same as a novel or poetry. There is a trick though I found -- If the kid memorizes the lyrics, then no copies are made. I never found anything against memorizing music or lyrics, although really it's just a copy in the mind. During lessons or classroom work, it's possible to focus on memorization techniques or 'studying' the form of the piece in terms of lyrics.... That's a transformative use, so anything printed for that doesn't count as breaking copyright since it's a different form than the original, it's studying it/discussing it but not using the material as it was originally intended, etc.
Looking at the original post again....
Sounds like you went off into teaching voice lessons during the piano lesson. I'd still just have them buy the music their working on. If they want to bring it to another teacher, so what? They own their own copy of the music then. Sounds like you gave them the lyrics (broke copyright then), but it's one piece from an anthology or some collection you own. In that case I'd say you don't loan out the book. You need the book for other students. Maybe it's just a 'demo' book for students to pick pieces from. Pick the piece, then the student buys their own copy.
Realistically, no one's going to care. It's doubtful anyone's going to turn anyone in to the publisher or copyright holder. It's good to mention the copyright angle and get the student/parents used to owning their own copy so it's not a shock in the future when it comes up.
It's possible the music is out of print (but not copyright) or actually out of copyright. You might be stuck if it's POP (permanent out of print) or orphaned (still under copyright but no one knows who owns the copyright).
Although if this is the piece, it's under copyright.
https://en.wikipedia.org/wiki/Tomorrow_(song_from_Annie)
""Tomorrow" is a song from the musical Annie, with music by Charles Strouse and lyrics by Martin Charnin, published in 1977. The number was originally written as "The Way We Live Now" for the 1970 short film Replay, with both music and lyrics by Strouse."
You could demand your lyrics copy back. Never admit that you gave that to the student. Then lawyer up and have the lawyer send a letter to the seven year, her parents, and school music teacher. Cover your butt and threaten to turn to them itno the publisher/copyright holder if proceed with their infringement practices.
Looks like her career never went anywhere in music or movies.
https://www.imdb.com/title/tt0083564/?ref_=nv_sr_1https://www.imdb.com/name/nm0703747/?ref_=tt_cl_t6Potentially, you could interpret the request a "copy" of the detail, like keypeg says. "Here's a copy of the information about the piece." Not a copy of the actual music, just the info on that exact version/arrangement.
Potentially, there's the arranger of the music too, another creator of the derivative work.
If it's Annie, though, it wouldn't be that difficult to take a guess on the music. Off the top of my head, I'd guess it's a Hal Leonard arrangement. If the parents buy the music, great. If the school teacher has their school buy it, great.
I would really guess the other music teacher is kind of being lazy. "Give me a copy of the music" because they're going to do something else with it. Have the kid sing it for something for a school performance. Maybe they're just going to have a class sing through it. What could really be bad form is if they're "stealing" your work with the kid and having that kid do a performance but imply that they (second teacher) worked with the kid for that performance.
I would just tell the parents/kid/other teacher that you can't give the book out but here's the info on the book. Kind of avoid the "copy" part. If they ask, tell them you can't just make a copy because of copyright law. The other music teacher won't question that at all. If they do, it's their problem.
If you really play it well, it's a chance to make a professional connection with the other teacher. If it's a public school teacher, they might be looking for an accompanist... In that case, if you did happen to break copyright law, you might have a happy teacher who would hire you for some accompanying in the future. Haha.