Perhaps what's needed is for the person who'se being treated like crap is for them to just walk away or file a formal complaint and keep their distance from the other person. There's ways to 'not stand for it' as well. If the person being treated like crap is going to yell, scream at or even threaten the person treating them badly then that sends a clear signal (despite it not being how the person wants to perceive them) to the person dishing out the treatment that it's okay to keep this going because they're getting a reaction. That's the biggest thing that keeps dramas like this going well past their used-by date. The other thing is that when we let someone know we consider them favourably - whether that's by wanting to be friends (or in the social circle someplace) or absolutely besotted and in-love, that gives them power because they have access to you. The majority of people wouldn't take advantage of that kind of power but then there are some who do. I see these kind of people as being abusive and cowards. I think you've hit the nail right on the head with the last sentence in your question: "What's their issue?" As long as the person being treated badly is able to protect themselves and stop reacting and simply practice the challenging yet still ultimately achievable act of keeping as far away as humanly possible from this individual and most importantly not showing the impact of the other person's behaviour, then it remains the other person's 'issue' and not theirs.

The real question is: Why are you still hanging around with someone who treats you like crap? He/She obviously doesn't want anything to do with you.