Mean girls

My sister has been obsessing about her high school daughter who appears to have been dumped by her friend group (yes that is a thought!). My sister is close friends with the best friend’s mom and their relationship is effected as well. After a marathon weekend watching How to Feel Better I thought I’d be able to model this, but I am stuck. I know you are going to say that no one’s actions can make you feel anything, but how does a 15 yo get that?
C spent 1 day with best friend in 3 past months
T – I dont know why she dumped me and is excluding me from group plans
F – rejection, lonely
A – stay home, try to reach out to other friends
R – no social plans
Intentional model
C – same
T – I am not sure why E has pulled back?? Need help here
F – curiosity, ?
A – ask her
R – understanding, closure

As for my sister (and Bf’s mom)
C – D and I talk less
T – D hasn’t been a good friend, she knows Y is being excluded and she hasn’t helped.
F – resentment, anger
A – don’t talk to her
R – not talking

What would the intentional model be?

Basic question about conflicts – when we have conflict in a relationship, are we to just change our thoughts that it is all ok and never get pissed off? Seems unrealistic. I’m missing something big here.