I had this exact same problem with a client who is thankfully no longer a client anymore. They had some ‘advice’ from a friend saying how to go about doing things on the site and I warned them that if they followed their advice it would have a very adverse affect on their conversion rates.
I explained everything very thoroughly to them as to the rights and wrongs of what the ‘SEO expert’ was telling them and even sent them lots of information online so that they could see that what I was saying wasn’t just made up.
Long story short they didn’t listen to me, ended up getting the whole site re-designed in ASP and now their site is missing half the features that I had put into it. Seems that clients always listen to everything they’re told but never believe the person who originally created their site which was working fine.
Sounds to me like you need to bring in a second 3rd party 😉 to tell them what they should do and get them to tell your client that your way is the right way.
I’ve vowed to myself though that even if this client came back on bended knee to me that I will never go near their site again. Just can’t be doing with the headaches. They still owe us money for hosting that they haven’t paid and it looks like we may well have to take them to small claims to get that back. Something which I really hate to have to do but we need the money to be able to survive so looks like it’s going that way :-(
Good luck on this and I hope the client has a brain and listens to you in the long run. I’m totally feeling you on this one.
Best wishes,
Mark