Hi there,
I would delegate the issue to your client or at least ask them for advice.
As a real estate firm they should know how to deal with law issues and it would be a simple thing for their lawyers to send an appropriate request to the web-desing branch of that company. I'd believe they should be happy to help you with that matter, because it is a competitor doing this. In preparation I would find out which person at that firm (with multiple internationl offices) is the one responsible (webmaster and CEO or president - CEO/president are in the hot seat for liability issues these days) and which law aplies to their web-presens or is best to threaten with. sometimes a particular US state law can be very nasty. (Did someone say Oregon or California ;-) ?
By the way check the wayback machine:
http://web.archive.org/web/*/http://...roperties.com/
There you can see, that in Feb 2003 the link to the web-dsign was not there. May be it is a simple mistake and they do happily correct it.
You might want to contact the webmasters of the other sites and ask who did their design or who maintains their site. May be they are as fake, than you know it's not an oversite or typo.
As a last resort you could instruct your web-server to show a special page for referals from this particular link (for apache use mod_rewrite) and tell any visitor coming via this link, the truth about it (probably needs permission from your client). This case might trigger some legal action from their part, if they believe they are right. So you better have some insurance that covers your defense costs - although they should sue the client (who if smart wants you to indemnify them before he/she agrees to this course of action - in any case get their agreement in writing).
Good luck
K<o>