Facebook iframe working on one server, but not another

Hello,
I am trying to put an iframe landing page up on a client’s FB page, and when I upload it to my server it works great. When the same page is uploaded to his server, it won’t work at all. I did change the file extension from .html to.php, because it wouldn’t work at all with html, and I double checked that all the pathways were correct to his server and still no luck.
Could it be his server platform? Or does anyone have any idea what a fix might be? He doesn’t want to pay me to host it on my site, since he has his own.
Thanks for any help!