Well, there are ways to run the two next to each other, or even easier is one inside the other. For example; you create a sub-folder in your main EE installation and run WP in that:
ExpressionEngine --> www.domain.com/
WordPress --> www.domain.com/blog/
Your WP site would need to be configured to run the blog roll as it's home page. You'll be able to share CSS and JS assets between the two easily enough, but you'll have to manually update templates in both EE and WP when you make changes you want to match in your global (combined EE+WP) site.
I have personally imported WP posts into an EE channel before. It was a pain in the ass and basically required constructing a bunch of regular expressions to separate out different parts of the WP posts and input them in to corresponding EE entries and fields. It did require some manual review to fix little content issue imports, but it was by-and-large an automated import and not a full manual import. The viability of writing an import script depends on your use cases and also on the consistency of the blog posts.
You could, say, create a channel with matching fields to a WP post (title, url-title, featured image, content (as, say, a Wygwam field)) and just directly import them like that. Again, depending on the blog use-case and consistency, this is possibly viable.
Another possibility is to use the
query tag to query the WP posts in a special template for the blog. But this is barely scratching the surface of the complexity of creating a template group that scrapes from the WP posts table. But I see this possibly doable. It also depends on how complex the WP blog is (WP posts are flat, or are there all sorts of categories a post could be tagged as? How about file assets?).
Overall, I would personally recommend writing an import script. You say
hundreds of entries; that's pretty easy to review after a script import. If it were thousands than I'd not consider it.
If import really isn't an option, running WP inside of a sub-folder of the EE web-root is your next best option. It will require a lot more maintenance if the site is to have a global matching theme and header/footer element structure.
Past that, I wouldn't touch it. Why is the client moving to EE if they don't want to utilize it's advantages over WP? And the legacy maintenance of watching a single blog... and I haven't even touched the worst part: are they going to want to write new news blog entries in EE, but still have them mixed with the old WP posts? Not happening (welllll sure, doable, but crazy amount of hours writing a very complex custom module to mix the EE Channels Entries tag with hand-written WP posts reader methods and... oh god, WHY?).
Oh and if you wanted to run WP on a sub-domain and just import rendered views instead of moving the installation to a sub-folder, you can use
mod_proxy. But this is another rabbit hole; use a sub-folder for this option.