My GM buddy told me that if any multiboxers enter an arena a GM is required to watch
Either this is a misunderstanding of what your buddy actually said, or there are different guidelines for EU GMs compared to USA GMs. Or your buddy simply got it wrong.

I don't doubt that some (most?) get a kick out of warching multiboxers in action, but there's absolutely no requirement (at least in Europe) for ANY arena match to be manually supervised in any way, shape or form.

I keep hoping this thread will just disappear in the noise, but it keeps popping up again. As others have observed, the single quickest way to kill multiboxing would be for Bliz to say "it's not allowed".

Most of us would stop immediately.