They have a single receptionist who answers the phone for everybody. Each tenant has his/her own office arranged around the area.
It's not uncommon for professionals who have similar needs to share office, work, or studio space within suites in commercial buildings. I've known many attorneys and web developers, eg, who've operated out of such arrangements as you describe above, and in fact have never heard of them called virtual offices before.
Often, attorneys who share such spaces share a law library. Web developers might share a very fast line to access the web. A group of photographers might share a studio. Quite often, in these arrangements, there's a shared conference room.
...and access to office space twice per month.
This, though, is where it gets confusing to me. If your client were actually working there, this would be a much bigger commitment to a business location than working at home, as it would definitely be a place of business.
But, if as you suggest, your client is just getting use of the address for, say, two days out of twenty days a month, you've possibly multiplied the number of tenants by 10, something that Google can track if enough of the tenants are registering this as a place of business (especially if a place of business with Google).
I've always felt this "address-crowding", to coin a term, would be the downside of using mailbox drop addresses to try to fake a location... that Google would notice an unnatural number of "suites" at an address. You'll see, if you search a multi-tenant business address in Google Local, Google will try to return the occupants of a building. So far, they aren't doing a terribly good job of it, but they get better with each additional bit of data.
As to whether a two-day a month virtual office is legit in Google's eyes, I don't know. Possibly, if enough real home-office professionals are using virtual offices as an arrangement because they need to have meeting space in an office building, then such an arrangement might be OK with Google too. If it becomes a common method to spam Google Local, then it won't continue to be OK.
An added complication, in all this, btw, is that your client's service area is not confined to the nominal business location even under this arrangement. The last I heard, which was a while back, service area in Google Local was an iffy thing. Perhaps someone with recent service area experience could comment on that particular aspect of this situation.