There are a couple of things I can think of off the top of my head.
1) Short the voice coil out by pushing too much current through the speaker, this also causes massive heat, which the speaker might not be able to dissapate properly. This is usually only do-able in subs since other speakers don't require as much power.
2) the surround comes unglued or dust cap comes unglued. This is caused by pushing the speakers way too hard for an extended period of time.
Now, unless your "friend" is willing to replace your blown speakers I wouldn't try this just for the hell of it.
Also, if you try and push alot of current to your sub through your amp..make sure your wiring is up to par, or else you'll have a big mess.
What are the specs on the things you are trying to blow up? Might help in determining if you can actually do this.