Personal Piece of Advice: Do NOT share your keystore with anyone
Other than that, you could have the client send you the apk, sign it, and send it back to him. If you don't want to do that, you will have to share the keystore to come to an amicable solution.
You could try to explain to the client the security risk it poses, and see if they agree to republish the app before even more ratings and downloads happen.
Additionally, you should always create a new keystore for each individual client (or even individual apps).