If you knew someone at work might be getting fired soon, would you tell him?
Definitely not.
The only way I could know for certain was if I were working in HR, in which case it would be privileged information. If I didn't know for sure then saying that sort of thing could create all sorts of issues, especially if it turned out I'd misunderstood.
The only time I've done anything like that was years ago when I worked for a small company and saw the accountant's job advertised in the local paper. Naturally I assumed he had resigned, so asked him where he was moving to. He didn't know what I was talking about, which created a rather awkward situation. To make sure I hadn't missed the point we went and bought a copy of the local paper. Truth be told in that particular situation I sided with the accountant rather than the company, because advertising for a position while the current occupant of the position thinks his job is secure is so underhand I regard it as extremely unethical. As it happens I left the company (by my choice) a few months later.