This forum is for Christian advice. I wonder about the same things, too. I feel I have been dumped for a variety of reasons. When it happens, I usually don't know what hit me. It just occurs to me what has happened.
That's one approach to take: do nothing hurtful directly to the person or indirectly by spreading gossip or such.
A bit stronger approach is to communicate with them distantly, such as through email (I suppose) and just bring up the issue, in a general way -- if there has been obvious abuse, this may be pointless.
If the relationship has reached a destructive phase, then it's time to back way off and fill up your time with something else. If someone was trying to teach you a lesson or communicate something to you -- your silence will reflect that you got the lesson or communication.
I think this kind of thing happens all too often. But, you have to know the limits of a relationship / friendship.
In grad school, I used to help this woman (about my age) with copies of class notes and gave some assistance with our work assignments. Then, I had a need to do a role reversal and ask for some help -- I found out she was very resentful about the help I had given her. She had a sarcastic demeanor that floored me, after I had helped her a couple times (I had held nothing back from her).
All I could do was clam up and retreat. There was nothing to say to her, after her outburst. "do unto others as you would have them do unto you" didn't work here, not even close.
This is not easy, even with a shallow acquaintance. No telling what a "friend" will do.
Keep in mind that people act very immaturely and childishly. A friend once said to me, Crumpy, it's all personal. don't ever forget that.
Part of what I'm saying is that what you do may haunt you, as well.