If your clients DA is 100, it's probably hosted on a Wordpress subdomain (http://clientblog.wordpress.com/), right?
I would incorporate that blog into their existing site on their domain and work with WP to redirect if at all possible. In terms of theory, this is pretty easy to think about. Simply ask yourself (or your client)...
"If a potential customer is taking the time to read this piece of content you took X minutes/hours/days to write, where would you prefer they read it? On Wordpress.com, where they can easily find content on the same subject from 1000s of other writers? Or on your site, where they can learn more about only YOU and YOUR business?"
If they can't wrap their head around the virtual aspects of Wordpress.com vs. their own domain, give them a real world analogy.
- Compare Wordpress.com to them having a pop-up stand at a tradeshow for anything and everything available for sale in the world, then
- Compare their own domain to their physical brick and mortar store.
Where would they rather the potential customer engage with their brand? I think we can all come to an agreement on what the right choice is for that one.
I'm not saying it's a BAD IDEA to have a blog on Wordpress.com, but the content they write should probably be generalized about their industry and offer links back to their services where applicable. Their company-centric news and top content for generating traffic via SEO/inbound marketing should be hosted on their own domain.
Hope this helps.