a few options:
1.) have the student create a repo with the same assignment name. Add her repo as a remote, and push to it.
2.) she might be able to re-accept the assignment–ask her to try again in a new browser. (This works 50% of the time in my experience).
3.) she could also checkout a branch that you can push to.
More details here: Setting students up for code reviews
Let us know how it shakes out.