You need to cast Player to CraftPlayer, after that get the EntityPlayer with CraftPlayer#getHandler(), where you can return the player ping as int value.
((CraftPlayer) player).getHandle().ping;
You also need to add the spigot dependency (the API doesn't seem to include it).
<dependency>
<groupId>org.spigotmc</groupId>
<artifactId>spigot</artifactId>
<version>LATEST</version>
<scope>provided</scope>
</dependency>
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