Hi Shauna!
So glad you've written in with an important question. Here is how this works:
Virtual offices are strictly forbidden by Google's quality guidelines (https://support.google.com/business/answer/3038177?hl=en) which state:
Address
Use a precise, accurate address to describe your business location. PO Boxes or mailboxes located at remote locations are not acceptable.
So, Google is as clear as can be on this one and you should be sure the client is being educated properly about the fact that he is not eligible for a Google+ Local page for any location other than his real, physical ones. Rather, your client's eligible business model would be as a Service Area Business (SAB), if he has a single physical location from which he is serving a variety of other locations.
Now, how this comes into play with Moz Local is that we only want to push out real, physical addresses, too, and we are reliant on the data Google and Facebook show us about a given business for validation purposes. So, if your client has a real physical address, then this is what he can use to build citations via a tool like ours, or, simply manually.
One last important thing on this, specifically regarding Moz Local. If the business is listed as an SAB on Google, this will result in his address being hidden by Google. Moz Local cannot pick up or validate against a hidden address. So, for our SAB customers, the alternate validation route is to be sure the full address is published on the Facebook Place instead, as Facebook does not require SABs to hide the address of their physical locale.
Further reading on the topic of virtual offices:
Here's a recent case of a Google takedown of virtual offices:
http://www.localsearchforum.com/google-local-important/26844-busted-warning-attorneys-regus-non-compliant-offices.html
And here's last year's Moz Local Search Ranking Factors, which cites listing detected at a false location as the #1 negative ranking factor:
https://moz.com/local-search-ranking-factors
Hopefully, you can share these things with the client and help them to ensure that they are running a guideline-compliant marketing program.