[support] rescrit access to a subdomain

[Simone Dal Maso]

Hi,
I don't know if this is a drupal issue, so I apologize if this request is 
made in the wrong place.I have my website, www.example.com.
I create a subdomain and it is properly running, with help of domain module.
http://subdomain.example.com

this subdomain is a service for a customer that has his own website, not 
with drupal, but wants a particular service and I make it for him. He wants 
that:
http://subdomain.example.com
is accessible only from its link in his homepage, not from external user. In 
a few words, the subdomain should work only if the visit comes from his 
website.
Do you think that a similar thing have sense?
I still didn't give any answer about it, because it appears to me a strange 
request and I don't know how to realize it.

What do you think about it?

ps: I really thank  this group because every time I made a question I always 
had a good feedback, sometime I forgot to thank people.