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If it's monetized based on page views then serialize it.

If it's for link bait, it's riveting to read and highly relevant then take a chance and put it up intact. I think you probably already know if it's good enough for this.

Definite plus one to that.

I don't think you can "tell" Google that different parts of your site are targeting different locations. If you were using multiple domains you could use the targeting section of GWMT but unfortunately that doesn't apply here.

There are ways that you can send a strong signal to Google that content is targeted at a specific country such as;

1. Use of local language
2. Using local currency
3. Local addresses/phone numbers
4. Links from other local sites

It might also be worth using the markup for places/organizations on Schema.org. I'm not sure if Google are using that particular microdata but it can't harm anything.

I worked on a couple of a sites in the travel sector that did this a few years back and it was fine. I wouldn't foresee it being a problem today either (although if anyone has evidence to the contrary I'd love to hear it).

I'm assuming that there are reasons why your client can't just create his City B content in a sub-folder of his City A site. Maybe the name of City A is in the domain or something?