Yeah, I agree with you – in this world, nobody is truly unbiased. I have been in situations similar to the tenant’s, and it really sucked.

I definitely had a knee-jerk reaction (deliberately inducing information asymmetry is kind of a hotbutton of mine; I prefer to be as open as possible even when I would derive advantage from not doing so.) That combined with my identification with the tenant, and ascribing to them my many positive attributes (poking fun at myself there!) drove me to inform them.

Will I do it again if I see a similar situation in the future? Maybe, maybe not. It would probably depend on how much I could find out about the situation. If I were inclined to, I think I would call the real estate agent first and ask about the listing. If they had a reason that I believed and considered ethical, I would forget about it. It probably makes me a hopeless busybody, but I still emotionally feel like I have a duty to act when I see something I think is wrong.