Why did nobody answer the phone when the customer discovered the leak?
I have a mobile dedicated to my customers for after sales service and of course future business with them, when it rings day or night I know it's someone I did a job for so unlike the other phone it gets answered 24 / 7.
If you had been first on the scene you would know if it was a faulty fitting or your fault, either way your priority is to limit the damage to a minimum.
I would fix the problem, clean up and when the property is back to pre-damage condition let the blame game begin, looks to me like someone dropped the ball where customer communication is concerned.
Most carpenters I know have general contractors insurance, I have it linked to my my policy as specialist heating and plumbing contractor it doesn't cost much extra and covers a multitude.
From what I read here you were a day labourer doing a bit of plumbing, since it's your brother give him back the £400.00 everyone has to learn from this, next job he does he will make sure everyone has proper cover to protect him and the customer.