Orangey, Randomnity, Protean and cascade have made all the points I would have attempted to make, but I'll add this:
It greatly bothers me that the ISTP's venting is being characterized by Orobas as "cruel." Saying a few unkind things out of frustration doesn't even begin to approach what I would consider to be cruel behavior. That is a bit dramatic. Cruel behavior would be giving them hope and taking it away on a whim, just to watch them suffer. Cruel would be telling her to her face what a horrible mother she is, and berating her for not making a better life for her children. Cruel would be letting them stay on the streets, in the cold, with no place to stay, and small children to house, knowing that you could provide shelter for them, at least temporarily. Maybe "cruel" is a slight exaggeration on Orobas' part...? Maybe like some of the ISTP's rhetoric was in her vent session? If so, it should be a little easier for Orobas to understand. Because if she really sees the venting as "cruel," it must be unbearable to live in this world for Fi.
The post you took so much offense, to, Orobas--Orangey's post--made some very, very good points. Why is it so much better to have warm, fuzzy feelings about this woman, than to actually DO something that helps her out, that gets her off the street, that puts food in her children's mouths? Why does the ISTP get painted in such an unkind light? She's actually DOING something. Venting is her coping mechanism. Why fault her for it? Why not see it for what it is?
Something else I've noticed is that the moment someone professes to be a Christian, people will come out of the woodwork to judge YOU for judging others, or for not being absolutely perfectly Christlike at all times. Christians are human. The Christian life is a journey, a lifelong process of trying to live according to Christ's teachings. Maybe the ISTP is working on herself. Maybe she extended herself in an act of charity, trying to do the right thing, but it's a challenge for her to deal with the repercussions of other people's behavior? Maybe she needs to work on that, and maybe she knows she needs to work on that. Maybe she's working on that, and she'll work on gossiping later? Christians aren't perfect, but many are really trying to do the best they can. That's something you won't know unless you're inside her head, though.