Why does my best friend only bother with me when she argues with her boyfriend?

I think you just answered your own question. She does only use you for comfort when she doesn't have her boyfriend, and that's wrong... I say you leave her alone. It's not good to be friends with people who are negative and wish washy. Continue to hang out with your group of friends and enjoy life. Don't let one person bring you down, especially if she probably isn't your real friend. So relax and do your own thing, okay? Good luck.