He probably could but it would more expensive and time consuming than would be worth it.
My general rule is never put anything up online that you wouldn't be prepared to lose. Has he thought about making a registration only message board instead if he must have them up somewhere. Yahoo groups aren't particularly secure.
It all depends if he can prove it's his. He could email the web host or whatever (sorry, I'm not technical) and tell them what's happened and they might remove the page. Does the original one he posted have a time and date stamp on?
Or you can give us a link and we can all go and comment on the thief's blog