My customers harddrive died when i finished fixing it what should i do?

Ok so im a sort of freelance computer repairman im in highschool and i needed the money so i started doing this sort of thing about 3 years ago. I have lots of experience with computers and stuff like this and up until today ive had no problems fixing everything that was thrown at me but i guess somethings bound to go wrong eventually. So heres my problem a customer came to me with a macbook that had gotten a virus (yes they do in fact get viruses), so i take his computer and give him a price he agrees (i dont charge much)and leaves the computer with me i put it off to the side and continue work on another computer when i finally get to this computer i turn it on and begin to work and succesfully get rid of the virus. I always double check my computers before i give them back so as i was checking this computer it freezes. I think its nothing and just powercycle the computer but it gives me the folder with a question mark icon(this means the computer cant find the system to boot from) so i pop in the osx install disk and go on disk utilities and theres no harddrive showing ... Im screwed i reboot and check twice more and nope so i figure the harddrives dead (it happens to all harddrives but why did it happen when i had it ?!?!?!) i doubt the guy has a backup and i myself cant afford data recovery he is a friend of my dads and my dad was who suggested he bring his computer to me for repair, now nobody knows this happened yet but i dont know what to do so basically im asking for advice on what i should do. Or how i can explain to the guy that it wasnt me who messed up but rather it was a thing of bad luck the guy is completely computer illiterate so he will doubt me if i tell him all this technical stuff he dosent understand help please :( thanks for reading this hopefully i get usefull advice