I think the thing I find difficult is the line in the OP 'Ruining her Christmas magic forever' ... so the implication that the ONLY thing that makes her DD's Christmas special is a belief in Santa.
I have encountered this - occasionally - in RL: the idea that once a child doesn't believe in Santa, that's Christmas done for: nothing fun or nice about Christmas will ever happen again - and in some cases, families actively create that situation, by working incredibly hard to 'keep the secret' with increasingly elaborate schemes, then basically dropping all Christmas traditions as soon as the child 'knows'.
We have never specifically fostered a belief in Father Christmas - yet we tell the stories, sing the songs, have stockings, give one another gifts (the DCs are old enough now to join in the present giving as well as receiving, and love that), have ritual menus, wear silly hats, laugh a lot....
I know this isn't much use to the OP now, but perhaps if she reassures her DD that everything will be lovely still, that there will be lots of special things that she can now be 'in on' [stockings for older siblings or whatever], then it will soften the blow?