There was a huge bougainvillea, in full flower, that had fallen off a pergola some time ago, and was in a lovely sprawling heap on the floor.

I called in to the house today. Horror!!! it was gone! a foot high stump is all that remains, I nearly cried.

I am sure he knows what he is doing, because he got the plant, and another one, that big, and it has obviously been cut back this hard in the past, there was a 4-inch thick stump that looks like it was cut back years ago. The current main branch that has just been cut was about 1.5-inches thick.
It is planted in the ground.
I don't have enough greek to talk about it with him.

Will it grow back?
How long before it flowers again?
What can I do to help it, water fertiliser etc.

I am a great armchair gardener, but tent to kill real living things!

We also have bougainvillea here in Spain & everyone cuts them back from time to time.It will most certainly grow back of its own accord.It needs to be secured to a sturdy support & you need to trai it with a firm hand.Take care though as it has some fierce thorns on it.Remove ant pale "water shoots" which occur.DON'T OVERFEED or OVERWATER.If you do the flowers will drop off.Bougainvillea's tend to search for water underground like a lot of the Med plants.
In my book on gardening in the Med it advises pruning back hard in January,
Good Luck
bettyespana

I certainly agree with the advice Betty has given you. Have faith, it will come back in time but possibly not until next year.