I know that FM7 supported the separation model but did FM6? I have a client who wants to revise her application and having the layers separate would be nice. She may eventually upgrade to FM8 but could I implement the separation model with her FM6 application?

You can you separate the data and the interface in FileMaker 6 but it's not nearly as elegant. The big difference is you can't base a layout on records from another file (table) like you can in FileMaker 7 or later. So, what you have to do is use portals and related fields to display data. In other words, you can only get a list view using a portal. In addition, once you scroll a portal in FileMaker 6, it locks the first related record making it impossible for anyone else to scroll the portal. Makes the separation model fairly useless if you can't have multiple people scrolling the same portal at the same time.

Thanks ... I guess I'll have to make the changes and then import the current data from the production database unless I can persuade the client to upgrade (which I have been told emphatically in another thread that I should do.)

I believe upgrading is very important but be sure you research the matter before you tell your client. FileMaker 6 and 7, 8 and 8.5 are practically different programs. While you can simply convert a solution and with very little work make it function the same in FileMaker 7 or later, you really should rethink the entire solution. I usually recommend converting and getting to work and then consider rewriting the solution from scratch.