Hi
I would think that renting a Regus office address is no different to renting office space in any serviced office location.
It will always take a bit of time to establish a new address/location and be able to SEO around it, but name, address, phone number (with the local code prefix) on the site page(s) is an definite requirement.
Provided your client is not going to be there for a short time then I would back that up by growing citations of their office so that all "signposts" so to speak point to their legitimate office location.
Rand Fishkin spoke on this subject in a Whiteboard Friday back in May. You can watch it here: <a title="http://moz.com/blog/discovering-local-citation-opportunities-whiteboard-friday">Discovering Local Citation Opportunities - Whiteboard Friday</a>
I hope that helps,
Peter
