There seem to be no issue on zip file attachments at the moment. May I request that you ask your client to click on this icon on the upper right hand corner of your messages window with him? Clicking on it will automatically show the files that you have shared with each other.
You can send me a private message with the details of your client so that I can try to replicate the issue for you. Alternatively, you may also open a ticket with the support team, together with screenshots of the issue so that they can escalate the issue with our engineers.