IANAL/IANYL. That said, you two have made this code available to the world under Apache 2, and it is generally held that licence grants are irrevocable once they have been relied on.
I would not do anything further under the existing repo. I would fork that repo as soon as possible - the forking being a clear point at which you're relying on your co-contributor's licence grant - and continue work on the forked project. You don't need to change the name, but it would be helpful to avoid confusion for end-users if you were to do so.
Your co-contributor might well insist on having his/her code removed from the original repository, and indeed it might be simpler to delete it. But it seems to me that (s)he can't insist on his/her code being removed from the fork (though you should be punctilious about preserving his/her copyright notices, as good manners, intellectual honesty, and the Apache licence require).