Relationship issues

Help.

So this story is from on of my mates. Could you guys help her out?

Ok so let's call my friend Jaylen. So Jaylen has been talking to this girl for a few days and they get really into it and start flirting and it gets sexual very fast for them but after a week the girl who we will call is Katie. Ok anyway Katie comes out and says she's in a relationship and Jaylen respects that until Katie says she's open for a poly relationship, and hours later she says it's a joke but continues to flirt with her and then add Jaylen to a group chat with Katie's girlfriend who'll we'll call Mia. So Mia and Jaylen get to know each other but don't know how to feel about a poly relationship considering how young they all are (ages are 13. 14. 15) and now Jaylen is stick and doesn't know what to do, please comment on what she should do.