All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
You should not have rented out the car without the permission of your friend. Your unauthorized disposal of it in such a way is what Muslim scholars call Ijaarat Al-Fudhooli (unauthorized agency). This transaction is considered a binding contract only if the owner of the property consents to it, according to the preponderant opinion of the two scholarly opinions in this regard. It is the opinion of the Hanafi and Maaliki scholars and one of the opinions reported on the authority of Imam Ahmad . Accordingly, if you informed your friend of what you did and he gave his consent to it, you are required to pay him the rent money that you received in full, whether what you paid to the woman or what is left with you, unless he waives his right to some of that money. If he does not give his consent to what you did, then you shall guarantee to give him the benefits obtained from his property, whether they are equal to or more than the rent money you obtained. If there is any damage caused to the car, you shall also guarantee it (pay for fixing it).
The fact that the car owner is a Hindu does not allow you to dispose of his property without his permission or to betray his trust. Instead, a Muslim has to demonstrate the Islamic morals such as truthfulness, loyalty, trustworthiness, and so on, in order to invite people to Islam by his actions before his words. He should not be a source of Fitnah (misguidance) for people nor drive them away from the religion of Allah.
Allah Knows best.