Wow, I have the same "problem". My husband actually agreed to go to Disney World next year and I have to decide when to go. I LOVE going to WDW at Christmas because I LOVE the decor and all the activities. My husband suggested that we go in Sept/Oct to try all the Halloween activities and the Food and Wine festival.
I know I am not helping. What I am doing, since I would be going this time next year is to see what is occurring in the parks right now and then closer to November/December I will look and see what is occurring then as well. After I collect all my "data", I will sit with my husband and see what he would enjoy as well. Since we have been to WDW during the Christmas season three times already, I am probably going to go during the Sept/Oct time period.
Sit with your friend, after you have collected your data, and discuss what occurs during each "season". There is plenty of videos on
YouTube and information on this website. It sounds like your friend would like the Sept/Oct season since she likes Halloween. You could always go during the Halloween season and then make future plans later for a Christmas season.
It is such a hard choice. But in any case, you and your friend will have a great time because you will be going to Disney World.
Just a note, I would go during the Christmas season, if I could.