G-DEFY is owned by a Portuguese, Pedro Silveira, the Managing Director of Heliportugal, and a very wealthy man, most of his money coming from property. I believe his wife is French and he spends a lot of time there. The boat may be French or Portuguese - the French flag may well be merely because it is visiting a French port, as is normal.
Pedro Silveira and his company are also mentioned on the What's New In West Africa thread on this forum; they're presently leasing a Dauphin SA365N and an Ecureil AS350B2 to Caverton Helicopters in Nigeria.
It's a pretty simple exercise to land the helicopter on the yacht. In most cases helicopters landed on relatively small yachts such as this are only flown to or from the yacht when it's moored in calm seas or alongside in harbour. They're merely kept there for use when the yacht is in port and having them tied down like this enables them to be taken around to wherever the owner is next sailing. This is probably true of Pedro Silveira as he likes to fly himself.