It is basically 24 hrs., it's 1 day....out of all the thousands of days you have been around. Within all those other days you did what...you enjoyed friends, you enjoyed family, you went on trips, you saw things that you will always cherish.
It's 1 day and as much as she is this or that, she may be crude, she may me self centered.....what ever she is she asked you to go with her on something she inside herself may be very afraid of/about....she chose you for a reason (I think)....The day will come and go, she may well go back to her all alone (by choice?) life, you on the other hand will go back to your enjoyed lifestyle as well.
Do this thing, be glad you were around to do it.....it's 1 day, it's 24 hrs.......I think she may be afraid of what's happened, happening to her. This is not something to complain about IMO.....even though she sounds like a dis-likable type it's something that a person has asked of you and like I said it's 1 friggen day out of thousands....