Ugh, I encounter this attitude from people a lot in my TNR work--"if you didn't feed the cats they wouldn't be here"; which I respond to by pointing out that no one would randomly begin putting out plates of cat food for cats that didn't exist. Honestly, it's like these people think you can chum for cats!
The City of Toronto has a TNR program and takes the stance that the feeding and sheltering feral cats on your own or public property, in combination with undertaking TNR efforts, is perfectly legal and not something that anyone can stop you from doing. If there is public property near by, and your municipality has a similar bylaw or stance on the feeding of community cats, I would feed the cat there. If so, there is nothing that your boss or co-workers could do to prevent you from doing this, and any disciplinary action on your boss's part would be inviting a lawsuit. I wouldn't suggest taking it nearly so far if it can be avoided, but dealing, as I do, with these sorts of situations on a fairly frequent basis, I know that things can escalate. I know of people who have been given eviction notices by their landlords for feeding community cats, believe it or not.
In any case, I would contact a local TNR group or Alley Cat Allies chapter to see what your options are and what your local bylaws are, as pertains to the feeding of community cats. That way, you'll at least be prepared if someone at your workplace decides to make a big stink.
Of course, the preferable course of action would be to rescue the cat if you suspect that he's lost or abandoned. Any vet's office or shelter should be able to scan the cat for a microchip. You can also try putting up found cat posters, if you think he might be lost. Again, a local TNR or no-kill cat rescue might be able to suggest a more specific course of action or help to rehome this kitty if his owner can't be found.
Your coworker who felt the need to tattle to the boss on you about something completely un-work-related sounds like a really special kind of loser. I'm always amazed by the number of adults working in professional offices who seem to have the mentality of obnoxious 12 year olds.