There are two issues in this question:
1. Ethically, what does the Bar require?
2. Practically, how do you set up a law office in Florida?
Ethics Department Informational Packet - Interstate Law Firms contains the ethics opinions relating to a "foreign" firm operating a Florida office. The principle point is contained in Consolidated Opinions 77-7, 77-9 and 77-10 which says that the interstate firm's Florida office must be operated by a resident Florida attorney who is a bona fide partner in the out-of-state firm.
I suggest you have the attorney who is planning to relocate to Florida contact the Florida Bar's Ethics Hotline and discuss any questions which might remain about the ethical viability of the proposed branch office. The Ethics Hotline number is (800) 235-8619.
With regard to the opening of the branch office, the Practice Resource Institute’s (PRI) webpage has a wealth of helpful information, including but not limited to the New Law Practice/New Office Checklist which will lead you through the recommended steps. As the checklist suggests, the Florida Bar member attorney may be able to take advantage of the member benefit provided by Corporate Creations. This organization may be able to provide significant discounts and important guidance in the proper set-up and registration of your firm's new office.
Also, read this article which discusses the types of entities available to Florida firms. Florida Statutes Chapter 621 covers Professional Services Corporations and Professional Limited Liability Companies.