Alternative POV: that's the worst time. At those moments, the person is unlikely to be very receptive. They'll see it as proof that you are a bitter, vengeful person not worth listening to. Yes, that would a bit hypocritical of them, but that's humanity for you. That hardens people, leading to exactly the kind of cold-hearted comments the original commenter was talking about. Also, you will have foregone an opportunity to show contrast with a better way of being around other people. Lead by example, and all that.

I suggest that the best time is a while after such an event, when they have recovered from it but it's still fresh in their memory. While they're feeling good about having received help, that's a good time to remind them that giving help also feels good. I've seen people's attitudes get turned around by that, vs. never by forsaking them in an hour of need.