Turn iMessage off to message another phone

Hello,

I’m in a bit of a dilemma at the moment, that even me, the dubbed “tech-savvy one” can’t get out of. I have a friend who bought a new phone recently. Her older phone was an iPhone, and she sent iMessages with me and another friend in a group chat. Now that she is no longer an iPhone owner, she can only view SMS chat. But when we chat her (iMessage on), since she uses SMS, she can’t view our group texts. When we text her individually (iMessage still on) the system think that she still is on an iPhone. But when we turn SMS off, I can’t text both friends because the other friend uses an iPhone. So every time I want to text my friend I have to turn iMessage off and can only text that one person. Is there any way I can fix this so I can group chat them? I’m not sure why Apple is not detecting the change when we text her individually.

[Re-Titled by Moderator]