Dear Konuk,
This issue is mostly related with the terms of the car rental agreement, however as a general practice the firms concerned do not accept no responsibility for any fine or payment for the hired car during the vehicle is used by the contractor.

As you have mentioned also that the firm should have got in touch with you about the matter,at least this is a requisite for you to be held liable in terms of law.(I do not know the friend of yours is a guarantor(bail) of your car rental contract, if so the treatment of the firm may be deemed to be true)

Anyhow the legal facts should be revealed by the court if the firm is eager to sue you relating the issue , contary to your statements as not having engaged in a police event.