Well, I see in the source several problems:
Most likely suspect: Set.rb - it includes the Set class from the Ruby Standard Library which is incompatible with SketchUp's own Set class.
There are modifications made to the core Kernel class, Object and Module. (And Fixnum and String) That's not including what REXML might do - it's quite a large library, and terrible slow! I think the Set.rb file is added for the sake of ReXML.
I'd suggest you contact the author and point out the incompatibilities the plugin causes. Post back link to this thread so the author can see the full story.
To the author or this plugin, should you read this - please have a look at this article I wrote about developing plugins for SketchUp: http://www.thomthom.net/thoughts/2012/01/golden-rules-of-sketchup-plugin-development/