Manage a client's Address Book

I would like to manage a client's Address Book list on my computer, but I'm afraid it will merge with my Address Book. Any tips? There is a doctor who I do some work for and I work from home. She wants to put her file in a dropbox on the cloud, have me open it at my home mac, and update her list. Do I need to use a database program for this? Maybe there's a way I can add a "user" to my address book for this doctor and manage her list that way.