While I hate seeing my friends treated badly (my best friend is currently in a weird online/phone relationship with a person who lives halfway across the country, and they've never actually met and this person routinely says things to my friend that are pretty horrible to the point that my friend gets visibly and verbally upset over it), I would speak my mind to them, but I would not dump them as a friend if I cared about them.
In my case, I told my friend straight out that this is weird and clearly causes a lot more upset than it's worth and is also damaging the chances of finding a real relationship with someone local. My friend was mad at me for saying so but eventually came around. We don't talk much about it now but we are still friends.
I couldn't imagine dumping a friend who was in an abusive relationship.