Hi There!
I would really urge you to explain to the business owner that sharing a phone number is liable to create problems for them in marketing the business. Their Google+ Local listings could merge, mixing up their data and adversely affecting each business' ability to rank. Moreover, if the two businesses are sharing a phone number, I'm guessing that they have some sort of call center that's trying to divvy up calls for one business or the other and Google expects each individual business to be directly contactable at its local number.
This might be a good thread on Linda Buquet's forum to show the business owner http://www.localsearchforum.com/local-seo-ranking/10508-business-2-offices-but-one-phone-number.html with an explanation of why no one would want to risk this kind of headache when a separate phone number should be an affordable and quick acquisition for any business.
If the business owner will not listen to this advice, then I would only suggest undertaking working with them if they remove all references to the phone number from the Internet for one of the business and solely promote contact information for the second business. This suggestions is not likely to be well received, because doubtless the owner wants to market both businesses, but it would be really frustrating to me personally to undertake marketing in this scenario knowing it could just create a mess instead of the good results I'd hope to be creating for the business.
On the Moz Local side, what you're seeing in the results is a pretty good illustration of the confusion a shared number can cause. While I believe you can list two businesses with the same phone number, I'm not 100% certain about this, and would recommend you contact the Help Team for further information on this. So sorry you're in a tough situation with this client - sounds like it needs some careful thought and a good chat with the business owner.
