The story meshes perfectly with my experience of reality.
For example, people who appear to be extremely wealthy are much less likely to be generous tippers, because most people with garish appearances are actually middle-class folk who are up to their ears in debt accrued while accumulating the trappings of their specious wealth.
The people who actually have money to spare are the ones who live within their means, whether that's tens of thousands of dollars per year, or tens of millions.
Similarly, people who seem like they have all the time in the world are actually less likely to be good samaritans, because they're caught up in the high-speed goal-oriented mental framework where every minute has a Scheduled Purpose That Must Be Fulfilled.
The people who actually have time to spare are the ones who do not over-commit themselves, and are "disconnected" in just the right ways. I don't mean disconnected technologically, because it's entirely possible to take a liesurely stroll while tinkering with a smartphone. I mean "disconnected" in the sense that they are not plugged into the country-wide network of social obligations and expectations that most of us accept without even realizing there is an alternative.
Also, from a more concrete standpoint, please consider the context in which your disbelief is thriving. This is a world where people lie to friends, business partners and loved ones for decades at a time to accumulate fraudulent wealth; where inexorable neuroses drive people to become serial rapists; where devoted hobbyists spend weeks and months of their lives to do things like catalog every instance of the "Wilhelm scream" in the history of cinema, or document the pop culture references in every episode of the Simpsons; where some privileged people are so affluent that they can spend more on a single meal than an entire family of third-world country residents spend in an entire year of living expenses. In this world, men can survive in the vacuum of outerspace, where there is literally nothing. Men can survive in the depths of the ocean, where the exponential weight of the ocean creates enough pressure to crush steel.
We live in a world of absurd extremes, pernicious and healthy. Viewed through this lens, is the OP story really so unbelievable? If Buddhist monks can forgive oppressors who murdered their friends and loved ones, is it really that improbable that some worldly Mexicans stopped to help a down-on-his-luck white guy because they had a jack and tire iron in the van?
Really?