The iPhone user has their group messaging or messaging settings set to use iMessage without SMS as a fallback, presumably. That's the only thing I can think of, and I remember some wonky group stuff happening when I used to use an iphone.
I think this is it. I am in a large group text with a bunch of people I met on another forum and one of the guys had this issue and had to fix it.
In a pinch it's both a recorder and music player for rehearsals or gigs.