maybe 3 / 4 or even 5 if they expecting to plaster to. plaster can take a good week to dry off if spread thickly. and you wouldnt want to tile on wet plaster.
those labour charges do seem steep though . shop around sara3.
Do not lend your friend £5000, without any kind of signed agreement stipulating when and how you will receive your money back.
If you don't then if your friend's business fails, he walks away and your are less 5k out of pocket. And don't think friends wouldn't do this....because they do.
Also, you would surely want to know if the money if being put to good use and not some nonense idea which will never work.
I would never borrow or lend that kind of money to a friend.