Hi Paulo,
Ok, so he is most likely looking at Visual Page Editor which is mainly Eclipse Public License (EPL) for the plugin code + Mozilla Public License (MPL) for the XULRunner part.
If he also needs the JBoss AS integration then we are talking LGPL, but I'll keep that out of the explanation for now - but just note it has some differences. You can see the license in each individual feature/plugin.
For EPL the best resource is http://www.eclipse.org/legal/eplfaq.php which explains the various scenarios.
With respect to "would he be required to make his SDK plugins available to the world as well" then if those are separate plugins and not derivative works then he can (basically) license them as he wants - see the FAQ for examples.
For EPL it's only if you modify (creates a derivative work) that the source code have to be made available, i.e. if you fix a bug in our richfaces plugin then you'll need to provide the changed code - if you write a plugin that provide support for new JSF components then you don't have to distributed the source code for those additional plugins.
So if that is all he is doing then that should work fine.
That above is all about copyright, on top of this comes trademark which is something I'm not the right to ask about. I can ask our legal to comment on this if you wish to.
btw. ignoring legallities a more important topic is what kind of API's you are going to use....we don't guarantee API compatibility for things out-of-the-box for *All* plugins - thus if you need something speak up on the jbosstools-dev list and we'll figure something out (patches welcome) ;)