A friend of mine used to be a few steps ahead of my in his career. I was very happy for him, and I saw him as a trustworthy person and a future leader.
He got into senior management in some startup, I was happy for him. We maintained a professional relationship when interacting for work, and we tried to avoid talking about work as much as possible during our spare time.
I got a new job, and he turned hostile. He congratulated with me sarcastically, stopped answering phone calls and started badmouthing me, turning times I asked him for support or advice into ammunition to slander me.
I know he is in trouble in his current job, as the mentor who hired him got sacked and he wants to leave. I sent a couple of high-paying jobs his way and gave him advice on transferable skills, and I never found signals our friendship was in trouble.
This concerned me in terms of managing my network. Assuming that it's normal to have people turn hostile just because you got a new job, how can one prevent trouble later on?
Never having people in your professional domain become too close? Always maintaining superficial relationships? I just don't know who to trust at this point, I have a new enemy and I did nothing to hurt him. I want to prevent this in my career.