Late response, but I ewould recommend what everyone else has given. From experience, if you were to send a flac to someone's phone number (iMessage), 60% of the time it decodes to m4a.
Depending on what iOS she has, (and iPhone), you can do a WeTransfer of the .flac files. If she downloads it, it would go into the "Files" app, where she can preview it from there. Storage heavy most likely, but should be okay.