Hi Richard,
I have a domain setup and the owner requires it to point to his other site that is hosted elsewhere. I did a ping on it's www address but if I point my DNS to that address, it points to a different site on that IP address.
Take note: *his* other site is hosted *elsewhere*. You cannot control "his" or "elsewhere". In my view, you do not have a problem. "His" has a problem, which either you or "elsewhere" can solve.
What to do?
Get on the phone with either "his" (your client) or "elsewhere" (the other company) and tell/ask either of them to fix "elsewhere"'s setup. "Elsewhere" needs to add a ServerAlias or something similar.
If I were you, I'd charge "his" for the bandwidth of having to handle every first request for that domain plus the time it takes you to get this done if "elsewhere" refuses to change it's setup.
BTW - You could always use a frameset with a 100% wide/high frame and one of zero pixels. go.to works that way, I believe. Scr*w old browsers. ;-)
HTH... Nico