The creepy pattern as I saw it manifested between a woman and a man who was preying on her both times. The third party could be any gender, but when I observed it, I was the third party. The man was a rapist.
The man had become closer to the woman over a short amount of time, so that they were now at Best Friends Forever levels of attachment, and he had edged out her previous close friends, particularly her previous close woman friends, and claimed a hell of a lot of her spare time.
She seemed edgy, restless. Eventually she said, in a fairly quiet moment, that she had rather a lot of heavy things on her mind but she wasn't sure who she could or should talk to about them.
I made the (logical, to me) suggestion that her new friend this guy was around an awful lot and was very close to her. And maybe there was an element of jealousy in it, because she was spending time that used to be our time with him, and wasn't being as forthcoming as before.
A long pause. Then, "There's him," she said, but looked evasive and unhappy, and dropped the subject.
Later, I would find out that he had been systematically isolating her from her support networks, and raping her.
This happened not once, but twice. Two different women. Two different predators. Two times that I didn't recognize the pattern until afterward when it suddenly clicked. If somebody has heavy stuff to talk about but the new BFF doesn't seem to be the person they want to talk to about it ... it may not be just something awkward. It may be something dangerous. See if it's you they want to talk to, if you have time and energy, even if you're not as close as you were before.