My friend has HUGE mood swings, HELP!?

My friend was going to go trick or treating with me, and these other girls, but she got really pissy, and know she still, is being pissy! What do I do, i really like her as a friend, but, well, she can be extremely mean, and moody. I want her to go trick or treating with me, but, I want an apology to! ANY suggestions, or comments, on what to do, would be greatly appreciated! Thank You!