A Coin to the Rescue
Question: I recently walked by an automobile parked near a meter and noticed that the owner had overstayed the time for which he had paid. A traffic inspector was approaching and it seemed certain that he would write a ticket unless I took the initiative of putting a coin of mine in the meter. What is the right thing to do in such a case?
Answer: In his explanation of the mitzvah of "Love your neighbor like yourself" Rambam writes that whatever you would want someone else to do for you it is incumbent on you to do for another (Laws of Mourning 14:1).
Since you, like everyone, would want another person to save you from the need to pay a parking fine, it would be proper for you to spend that little amount in order to save him a great deal of money.
This rule of thumb applies, however, to a situation in which you can assume that the car owner initially put a coin in the meter but was prevented by circumstances from returning to it before his time expired. Should it be clear that this was an irresponsible driver who didn't even bother to put a coin in the meter when he parked, there is no moral obligation to bail him out at your expense.
(Based on a ruling by Rabbi Yitzchak Zilberstein, rav of the Ramat Elchanan Community in Bnei Brak)