Hi randomsinger.
I didn't know you were abused by your father. I'm so sorry.
If you have been abused, it's common to have bad dreams relating to this. I don't know why you are having this dream now. It could be because of something that is stressing you in life now, or maybe it is around the anniversary of an important date.
Your father turning your friend into raindrops and making her disappear can be a sign that you want something in your life to disappear.
Rain can also be a symbol of cleansing.
Since you have been abused and you are having bad dreams about it for some reason I think you should speak to a counsellor or at least someone you can trust to share your feelings with.
Take care.