Just a moment to try to help here (thanks for continuing to understand the why, @ace ) while in between fun.
First of all, I’m not aware of any Lightning to Lighting cables being made, let alone blessed (MFi) by Apple, so I don’t know how any software can make a cliam of this being remotely possible without an intermediate device or cloud service; second, without casting aspersions, many of these vendors, as I’ve sadly discovered, use servers located in China and elsewhere that frequently require your AppleID and password to function, and, I don’t know about you, but that’s the line when my paranoia (read: common sense) kicks in.
Adam is correct; if all of the music was purchased from iTunes Store, AND all of that music is still licensed to Apple from the publishers, then it should all transfer down again when performing an iCloud backup and restore, at least after the restore, where you can request to d/l all the music after all the apps and data are done.
However, any side-loaded music (via iTunes or otherwise), or music obtained via third party apps (Prime, etc.) and added to the music library won’t make the jump.
This does not account for third party streamers, like Spotify, Amazon, Pandora, Google Play, etc., whose music is usually cached and readily re-downloaded; and, even if purchased, are also available to download a second time (but there are limits she may have already reached); be sure you confirm what, if any portions of that 12GB being reported is actually in the main library, and not just a cumulative number from all music/media apps.
One would presume that if she doesn’t own a computer, and never has, that her tunes are all purchased via iTunes, but I would never want to make that guess.
I’m going to guess that, short of the latter scenario, you’re just going to have to borrow a computer (Mac or Windows) new enough to support the latest iTunes that supports her new iPhone, long enough to create a unique user account, so that you can do a complete backup and transfer all that music to the computer, then back to the new phone. The account and all the data can be deleted after successful completion.
Yes, iMazing and other apps will also do this, and arguably easier, but it’s not necessary.
Surely someone would grant access to a machine long enough (probably 6-8 hours for 12GB) to knock this out; a local MUG or a fee to a local shop may be required.
Next, I would ask her how much her music (and photos, etc.) means to her, and if there’s any value, she needs to find at least a used computer to do at least one backup now and again (or just keep the old iPhone and never obtain any new data).
I will note that I’ve encountered more than one person in possession of a gifted device that came preloaded or was otherwise later stuffed with “gifted” (ahem) DRM-less music and movies, for which they had no real ownership, and I declined to assist, as I didn’t want any pirated files moving across my machines, but that’s just me.
The good news is she presumably can keep both iPhones for as long as it takes to get this done, or just keep the old one as an iPod and carry both.
Oh, one crappy way to get this done just occurred to me: I suppose it would be possible using third party apps to move a non-DRM music library onto Dropbox or Google Drive as proxy, but it’s not possible (that I’m aware of) to do this with iCloud (it may have slipped my attention that a swiss-army-file-manager app exists that could be finessed, but it would likely be one file at a time).