On the whole, speeding tickets issued abroad and sent to your home address can be safely ignored. Only where bilateral agreements between two countries regarding their enforcement exist, would it be wise to pay up. To the best of my knowledge, Germany has such an agreement only with Austria. There are plans to extend the agreement throughout the EU, but this has, as yet, not gone past the proposal stage.
However, you mention that your friend was driving a hire car. In this case, if he does not pay, the ticket will go back to the hire company and they will pay and recoup the costs from him. Presumably the rental was paid by credit card so they will bill the credit card, the details of which they will still have on file. This has happened to me (not that I was speeding, I may add. The car was picked up by an employee of the car hire company and taken from the hotel to the office and he was obviously speeding. However, it proved too difficult for me to prove at such a distance in a foreign country, so, for the sake of the 15 Euro fine, I coughed up and left it there.)