What mistake is it that this person should have changed?

Not to use any from of possible objectionable joke/language when talking privately with a friend but in a public setting? I guess he should never again say anything that has to do with politics, religion, sex, violence, or technical standards.

Or is it to avoid any places with people he doesn't know. Avoid all conferences, bars, cities, towns, buss stations and so on? Only ever be at places where he know he can speak freely without anyone taking "offense" if what he said is overheard.