New member having troubles please help!!

Hi all

My name is Kieran and I am on here on behalf of my dad who
lives in Spain and recently bought an Audi Q7. He bought the car in England and
drove the car from Newcastle to Benidorm where he lives without any problems,
however when he got to spain he put the car in the garage and didn’t use it for
around 5 days while he was at work and when he came around to use it the car wouldn’t
start and it had a dead battery. He jump started the car and managed to get it
going but when he left the car again the same thing happened the car wouldn’t start
so he jumped it again and took it to a garage to get it a new battery. He then
put the car back in the garage with the new battery on and assuming he had
fixed the problem by changing the old battery for a new one. When he came to
start the car 3 days later still on the new battery the car is really sluggish
to start and trying to turn the car on is draining the battery and stopping the
car from starting. We are completely stumped

Does anyone have any clue as to what this could be

Thanks in advance for any help 

Kieran