I would say breaking off a friendship is generally not a sin. However, as in many things, your motivation for doing so may shed some light on the issue. Sometimes we break off friendships because they are not healthy for us.
But let’s say that I break off a friendship because that friend is not cool to another group of people that I am trying to impress. So I hurt a lifelong friend because he’s a geek, or has a birth defect, or he’s just not as good-looking as this group I want to be part of - he’s holding me back. Could that be a sin?
Now, your second question is more troubling. I cause a friend to breakup with another friend. Again, I would have to question your motivation. Did you cause the break-up because you believed in your heart that your friend was better off without that friendship?
That friendship was going to lead to drugs or illicit activity that you thought would harm your friend - than your motives are good. You want to break up that friendship because you are jealous of it. I would suggest that may very well be a sin if your motives are not in the interest of your friend. (but because that is subjective, you must inspect your own motives and feelings about it)