If he really cloned games pixel-for-pixel, wouldn't the original authors of those games have a good copyright claim? It'd be hard to fight their lawyers if your case is iffy, but if the violation is as flagrant as you say, I'm sure the aggrieved could find a firm that would want to relieve Zynga of some of that VC money. Even if Zynga made some changes, it's likely the original authors would still be able to make the case that Zynga's product was a derivative work. The fact that this hasn't happened makes me skeptical of such claims.

If the copy wasn't close enough to be considered a copyright violation, then what's the problem? Everyone takes inspiration from others. We're all standing on the shoulders of giants. There's nothing wrong with deciding you want to compete with someone and making a similar product. Not every idea for a new company or product has to be (or even can be) completely original.