Weird Lead Tracking Question

I've got a somewhat odd question (probably).

I'm doing lead generation for someone who is passing these on to certain business
owners, and he wants to make sure we get "credit" for those leads.

The thing is that one of our key clients has a website that would probably convert
pretty well, but they're not very internet savvy nor very organized with dealing with
incoming leads, and we don't currently have access to their website's back end.

I could do some offsite SEO that would get their site listed so it will get more action,
but how would I be able to prove that this is because of our efforts?

Mostly we're building our own websites. But in this case, I'd like to know if there's a
round-about way to send the leads to their website (maybe through a gateway website
that captures the info or where things can be tracked somehow).

Sorry if this sounds confusing. I usually work with opt-in forms, which makes tracking
those leads VERY easy.

But maybe there's a way...

Thanks so much for any help.

Elisabeth