The Script Blocker module has to "sit somewhere". It must be connected to the browser such that it intercepts the scripts loaded by the browser, check them and if they are OK, pass them on for execution (if they are infected, block them somehow).
In the browsers supported so far, it's possible to do somehow (i.e. "hook" the scripting engine of the browser and stick avast! engine in between) - even though it's quite a big hack anyway. In Opera, however, it's simply not possible - the browser is build as one piece, without any interface that would allow such hooking. Don't know how to explain it better...
Anyway, I don't think your friend has to be worried much. The most common target of exploits is certainly IE; I'd feel quite safe with Opera.