Couple options for you OP:
1) Coworking/hotdesking incubator style joints are popping up all over these days. They are often single location setups, with a small membership, and often offer a mailbox rental service for as little at $20/mo. You may have trouble finding one in smaller communities, but most bigger cities now have them. PM me if you'd like an example. In my experience, they don't always advertise this service on their website, so a call may be in order. Another good thing about these is they often have meeting space that your can rent out a la carte. For attorneys this is important as most state BAR rules state that the lawyer must be able to sit down w/ a client at the location they are advertising.
2) Another local lawyer, who doesn't offer the same services as you or your client. I've known many lawyers who have flexed this option, and working out a deal to rent a small sliver of space from another firm w/ too much space, and the occasional use of their boardroom. This will cost you $$$ surely, but I can't think of too many more legit options than this.
Now, there is a common misconception I believe that using a location that has lots of other businesses sharing the exact same address is a problem. In practice, I have not seen this. If you phone, business name, URL, etc is different, I don't see the problem. Consider for a moment the many other industries where this would be a normal and expected scenario - there are a lot of them. The only issue I see is if one of those 100 businesses exploit the location hard, setup 100 lead gen, spammy whatevers, it could find its way on to Google's radar. Mind you, I think it should be understood that there is always this risk when using any type of virtual setup, so this is just a given.
With that said, citations are a none issue.
Been down this road with a lot of legal clients, please reach out if you have any other Qs.
-Adam