I think in many circumstances it would depend entirely on what s/he had done. What I mean is ... the severity of my response would be guided by the seriousness of their actions.
That said, I can't think of many circumstances where someone would behave in a discriminatory manner and I would think that it was a minor issue.
In general, if I had any reason to think that s/he needed to be disciplined about something then I would pass it directly to their line manager.
Also, although it might seem like quite a minor thing to you, s/he might have been reprimanded about it before (and you might not be aware of it.) In that case it would be very important for the line manager to know about it because clearly the previous poor performance has not improved.
If you are concerned about involving yourself, sometimes organisations have whistleblowing policies to use in this type of circumstance which can protect your anonimity.