Although the forum posters are correct about kibitzer's rights and they MAY be ignored by a TD if called by them, I think it's perfectly acceptable for anyone at all to draw attention to an illegality or irregularity, particularly if it appears to be ongoing.
If this means the person seeing this irregularity calls attention to it after the fact, since they may have no right to call at the time it happens, isn't that better than not reporting it at all?
We have a great many players in our games, some of the games are quite large, some of them are quite fast and some are both large AND fast. This means that a TD cannot see what's going on at each table all the time or even most of the time. The TDs rely on player's calls to adjudicate. The ACBL Coordinator on BBO relies on email feedback to attempt to fix ongoing or budding problems.
So, if the question was, should a spectator write to ACBL@bridgebase.com to point out what appears to be an ongoing irregularity in the bidding? Then the answer is: Absolutely. Just as someone seeing a violation of BBO rules on language may report the incident to abuse@bridgebase.com even though they were not directly involved in the incident themselves.
Jacki