Is this person the executor of the will? If so, I guess contact is unavoidable.
I had an abusive boss _ (so eight hours a day, five days a week, we had to cope) Sadly, it turned out he was mentally ill. I learned to work around him, ask questions of other people whenever possible, which meant saving *all* my questions and asking when Boss went to the men's room or out for lunch. At least I spoke with him a little less. I also had a mantra going through my head; "He does not sign my paycheck. He does not sign my paycheck." It did help.
I rarely answer my phone. When I return calls I try to have all my ducks in a row and conclude the conversation as quickly as possible. "The dog has to be walked/ I have company on the way / I have dinner on the stove / I have to get back to my Swedish death cleaning" would all be valid reasons to conclude a phone call
Can people be abusive of just one individual? Absolutely, in fact there are lots of cases of child abuse in which one child in the family was singled out.
If your addicted relative isn't the executor, you can have a conversation with the executor and tell him/her you wish to be contacted (emailed, maybe?) directly and not get information second-hand through your relative. That should happen anyway. You'd want things in writing, so the executor can bcc the email of each of you.