Financial & Legal
if you have their "numero siret" which should be marked on their devis or bills you can go on www.infrogreffe.fr and check their administrative status (ie if they ve gon bankrupt officially or whatever)
Feel for you as have suffered in the past from the same sort of situation. (& never did get our money back just to cheer you up ;-/)
Best of luck
If you have a problem, involving loosing money, fraud, etc then report it to the police in Monaco. I assume they will not ignore if a Monaco based company and individual is not following the law.
Or/and go to his address and wait for him.
It is public information that In the past few yearsJohn Olsen had two other construction companies, JO Construction, and JO Construction 2010, both are liquidated....