As long as you don't give him the files, he can install it. You would need to have loaded the files and then let him do the install. Unless he or she has their own license and access to the files. Also, if you don't have access to v5 then he/she can only install the version you have access to. You can set up your site as a sandbox on your main domain if you wish as long as you do not have any other install on another domain if you only have one license. You may still want a development site in a development subdomain for use to test things after your main site is running. If so, put it in a "development" subdomain for the licensed domain and have it password protected so the public doesn't have access. You are allowed one main live site and one development private site on the same domain as the licensed domain just so you know.