This may be the last update. I believe I had managed to unravel the mystery.

After countless hours of reading relevant information, ruminating, revising logs and consulting other people, I came to the conclusion that she suffers from the Covert version of NPD (Narcissist Personality Disorder).
What happened with my friend was probably just her procuring her new supply beforehand by separating him from, and putting against me.
Feeling candid now, I’ve realized just how much she manipulated, led on and gaslit me. Those things you ignore while blinded by love.

To everybody who’s currently suffering in a situation which doesn’t seem to be making any sense, where their loved one did a complete 180 on them, do read on the various forms of NPD, how they conduct their “relationships” and how they pull apart and triangulate your friends.