Hi Annie. So sorry for this mishap.
My grandpa always told me, "When you loose some bottom, loose as much on the top". He also told me, "Plants are engineered to reproduce".
My opinion would be to prune the tree. And the remaining roots could start growing. Now I have no experience with Tree Peony, but I did have a branch (a little over 1" diameter) get broken off a Bursera fagaroides. I pruned the top and staked the branch in soil and it rooted.
I wish you well with your tree.
