I need to do the following:
When a client accesses www.hisdomain.com/admin he actually goes to www.mydomain.com/admin and this process should be completely seamless.

So I put into client's admin directory .htaccess file with following lines:
RewriteEngine on
RewriteRule (.*) http://www.mydomain.com/admin/$1

Allrite, it already goes to http://www.mydomain.com/admin/ but all links and images at these pages point to www.mydomain.com/........ and I want to hide real path so everything goes to www.hisdomain.com. All links are relative and paths inside files are relative.

So how do I set up that? I guess I shoud put something in .htaccess in my admin directory.