In this case, the driver of the vehicle will have to pay the fine. The owner will generally receive this by mail a while after the rental has ended and will get in contact with us or you directly.
If you refuse to go through with payment, the owner can send your contact details to the police with the rental contract enclosed. The best way to go about this is to warn the owner that you have been ticketed and that you have paid the fine, or that you are willing to pay when the owner receives it.
Be aware of low emission zones, and ask the owner of the vehicle if you are not sure how to proceed.