sole proprietorship

Dears,
I have a work-related question. Is it possible for an EU national who registered his business in another EU country to run it (on) in Belgium without the obligation to register in BE as an independent?

What I'm asking about is a business providing intanglible services. This friend of mine is exempted from VAT.

I'm at a loss - I just couldn't give him any advice. Myself I wouldn't probably have the stamina to register as an independent here taking into account the red tape.
On the other hand, I would never encourage anyone to work illegally...
What do you think?