"Artists Beware"

I think, double check with the commissioner first and see if they want you to confront the other artist or keep waiting. If they want you to say something, then confront with proof of previous sending, resend, and demand they hold up to their half. If they refuse, artist beware the crap outta them. If the commissioner would prefer to wait a little longer, then don't worry about it for now. It's their money and they can wait years of they want, just hold on to screencaps to show later that you fully fulfilled your half and that you complied with their desires.

Regardless, don't collab with the other artist anymore. They clearly are unable to be professional about it.