If I disavow the spammy links, would this improve my clients listing again on Google? sounds ironic that he will pay me to remove them
"Ironic" is a good word for it. I'm very uncomfortable with the idea of a consultant charging a client to perform some service and then charging them again to undo it, even if there was a period of time during which the client benefited from that work. But maybe that's just me.
Jill, I've got a hypothetical question. I agree with you that you shouldn't use Google's tool to disavow links when Google hasn't told you that you've got links to disavow. But if you know that you've got spammy links, even if Google hasn't specifically told you that they're causing problems, do you think it's a good idea to make an effort to get those links taken down? Redux has already indicated that he tried to get at least some of the links removed and didn't have any luck, which is what makes this question hypothetical, but I tend to think that getting rid of bad links definitely ought to be one of the things you take care of when you first start to work on a site, and probably ought to be part of the site's regular upkeep.
The problem is that such a task has a pretty low success rate, and that's compounded by the fact that in some cases the SEO would be trying to remove links that s/he charged the customer to get in the first place.