Let's assume he has ftp set to write. FTP in, change to root, put a file. Now search for that file on his drive. You've just found the root to ftp. If he has write disabled, log in, change to the root directory, look for files, directories, etc. Then search on his computer for those and you'll have found root of ftp.
If he's using Windows IIS, start, programs, administrative tools, IIS manager, ftp sites, select one, right click and go to properties. It'll have the directory listed there too.