Q: Do I have a responsibility to spare my neighbor from loss?

A: If someone faces potential loss, you are obligated to spare him, even if it involves time and physical effort on your part. If hashavas aveidah requires returning items already lost, then all the more so we must protect the owner from suffering loss (C.M. 259:9; Shulchan Aruch Harav, Metzia #31; Pischei Choshen, Aveidah 1:20) Therefore:

If you see thieves breaking into a neighbor’s apartment, you are required to summon the police. Similarly, if you observe a hit-and-run accident, you should provide any details you have.

If water or fire threatens your neighbor’s property, you must alert him. The same applies if he left his water, air conditioner or light on inadvertently.

If a lender forgot about a loan, you should remind him. Similarly, if you can testify on behalf of someone on a monetary issue, you are required to do so (C.M. 28:1).

If someone is about to purchase something from an unscrupulous vendor who overcharges significantly, you should warn him (Chofetz Chaim, Rechilus 9:10-12).

If your friend is ill, you should daven on his behalf (Hashavas Aveidah K’halachah 13:1-6).