i think if she has already told her management she will do the shift, then it is unfair for you to bring up this conversation again. accept that what you did was unfair - setting her up for a fall, so to speak - and just support her until Sunday.

if this issue comes up again, then it would be wise to have a face to face discussion then. but i dont think it's fair for right now - i don't think she is doing anything wrong.