Here's the deal.
1) Take your father's money, and give it to the shop that did the work.
2) Let your GF work out with her father which of them is going to write you a cheque to pay for it - how about they go 50/50?
3) Take your girlfriend's (or her father's) money and drip feed it back to your father so he thinks it's you paying him back as and when you can afford it.
Obviously this only works if nobody finds out what the others know