For specificity let's say an acquaintance you don't know very well is dealing with a health problem (not terminal or debilitating) that they are undergoing treatment for and that you won't see them for a while. Should you wish them luck with their treatment or ignore it?
The pro of addressing the issue is that doing so shows respect to the person, but on the other hand it may be uncomfortable or saddening to the other party. Ignoring the problem may come off as disrespectful, though.
I understand that such a scenario probably needs to be determined on a case-by-case basis, what I am looking for is a general rule of thumb for making the decision.