Hmm from what you posted, it is clearly a fault from your friends side. He received the legal notice means Airtel warned him already - for sure. So he was aware of it isn't it ?
The postal bill didn't reach you, that is a valid point I agree, may be he can raise it too.
But Airtel usually send information on bill (min of two sms) and reminders after that. Relying on postal bill is never a good idea.
I never wait for my postal bill. I am an Airtel postpaid customer for the last 5 yrs or so. I always check it online and once I see new bill has come, I pay it online instantly. I agree if your friend is a non-techie, paying online or checking it online will not be possible. But bill payment of an Airtel postpaid connection is possible through all of their outlets throughout India. And many of the shops which sell the pre-paid cards etc started accepting postpaid bills.
So I do not think the excuse (non receipt of printed bill) will make any difference, but give it a try and they may consider it. As it reached the legal steps, not sure about it though.