How should a person interact with an individual who has unfortunately married out? On the one hand, if one would behave hostile, it may prevent them from ever returning to Yiddishkeit. But on the other hand, behaving in a friendly normal manner, would perhaps portray an acceptance of such deplorable behaviour?
You are correct, there is no one size fits all answer for this. It depends on the person’s relationship to the person, the reason they intermarried, the person’s history, etc. This has to be discussed with a Rov that is familiar with the situation.
Have a good day.