Maybe I am reading this wrong, because it really could easily be the case too, but I don't actually think Sonya is in love, the fact she says "please don't disappear from me too" seems to imply people leave her on her own/people have left her life.
Maybe her life isn't so different from Millie's in the abandonment aspect. So "losing" her best friend just hurts even if thats not exactly fully happening.
I do like how Sonya is being open and honest though but not terrible to Zen, shes acknowledging Millie and Zen have done nothing wrong and that she doesnt actually dislike Zen its just...shes feeling replaced.