It sounds as if your migration consultant is unaware that you do not host your forum yourself. Nabble does not offer FTP access to their servers.
Depending on the size of your forum and assuming that you have no private areas that require logging in to view, the best approach will probably be to use an "off-line browser" or "web site copier" software.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.