How to Create Templates for Windows Publisher on a Mac
I understand that there may be different ways to achieve the end goal but since no one has come up with a solution at this point, here is what I suggest you try:

There is a neat little application for the Mac called Wine. All in all it allows you to run Windows applications on a mac.
About 7 months ago, I used it to run Publisher on mac without a single hiccup along the way. Your client will save a bit of money with the InDesign app and (depending on the size of your project) perhaps it does the job better than any other third party applications.

Here is a link:
http://smallbusiness.chron.com/open-microsoft-publisher-mac-56433.html

Give it a go and let me know if this answer suits your project.