First things first. How much co-operation can you expect from the adult with the bad habit? Going with "a minimal amount but open to advice......." I'd say to give it a try. I'll go further and suppose the habit is ....constantly interrupting someone who is speaking. Very annoying habit, agree? With patience and genuine concern, help the adult recognize that he/she truly has this habit and it is apparent it has caused them some back lash. Give this person a few actual incidents when, where and with whom they have exposed this unacceptable behavior. As you walk them through, it would be important to ask them how they feel they might react, were they the person being continually interrupted. I think it may help quite a bit to offer to go through a role-playing scenario, one on one and spend some time discussing it afterward. No one enjoys hearing about a character flaw they may have and it's no fun pointing it out either. Being considerate and respectful should make this more comfortable for both of you.