I don't spend my life assuming that people wake up and decide to be evil or a bitch today. Generally they have their reasons for whatever ass backward thing they do, even if I don't agree with the reasoning or the result. But sometimes the reason doesn't matter so much as the affect their choices have on other people, especially if they are made aware and either don't change, or only pretend to have changed. But see, the other side of that coin is paranoia. Who is to say that they really haven't changed when you're looking for signs that they haven't, no real truce has been offered directly, and thus you're seeing things that might not even be there?
If person A decides that you are bad for someone they love (person B), and thus hostility and distance creep into a situation where there should at least be friendliness, to the point that it is necessary for person B to lay down the law and say it is not acceptable to treat the person they also care for in return (person C) in the hostile fashion they have adopted, it could be said that person A had not been called on their behavior before that point and thus there was a misunderstanding based on them wanting what is best for person B. But then, what if person A seems, to person C to be barely hiding their real feelings about person C from that point forward, offering very little real attempt to bridge the gap (even though person C has not trusted them enough going forward to work much for a bridge, either), and they seem to lie in wait for some opportunity where the tide might shift and their opinion will be allowed to triumph? Person A is not actively doing anything so much as not doing things that would seem appropriate to any given situation were there real friendship or forgiveness between persons A and C, in person C's perception. It could be that person A just doesn't know where to start to bridge the gap. But assuming this could also be dangerous to person C when there has not even ever been an apology offered for the previous negative behavior. It could be that person C is right and person A would relish the opportunity to trash person C openly and loudly with a triumphant "I told you so!" should the tide of person B's feelings ever change. But it could just be that both parties suck at bridges and the circumstances have lead to a strange and akward situation.
Generally in my history, I have decided that person A was not worth my time or emotions if I was person C, and I've done everything I could to avoid their very existance because I do not trust them, but in this instance, person B cares about both parties a great deal and wants to believe that person A has now changed and feels that person C is seeming sort of extreme for feeling like person A should be deliberately excluded from knowledge of vulnerable topics to person C out of self protection, when person B wants very much to talk to person A about those details. It is further compounded by person C having observed person A pushing very hard to destroy person D's relationships even after person A has been told to back off, and person A's methods have just gotten more subtle in their attempts to cause failure to person D's relationships, which has been admitted by person A to persons B and C behind person D's back. Thus person C knows person A is capable of that and doesn't like person B going to person A to talk about or ask for advice about anything relevant to person C.
Is person A abusive? This is impossible to answer objectively. Is it fair for person B to talk to person A about person C or person C to ask person B not to talk to person A?