When people ignore email messages, they can actually be sending an important message. Before focusing on your co-worker, take a look at overall email practices in your company.
Some companies place excessive reliance on email and use it in situations that do not lend themselves well to this medium. For example, there can be long chains of email messages filled with accusations, claims, counterclaims, and arguments. There can also be a mass of additional messages that are nothing more than copies that people send to almost everyone just for backside protection. The fact that people use email "all the time" in your company indicates that it may be overused and in need of review.
It is clear that there are many messages that are best delivered either in person or by phone, especially those that have an emotional charge or deal with feedback, questionable behaviors, or conflict. Email on these types of matters tends to exacerbate problems rather than eliminate them. In fact, it would be both ironic and futile for you to try to solve this matter by email. The best step is to meet face-to-face with your co-worker to discuss the situation and a strategy to resolve it.
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