I think this is an excellent question. I guess I try to take the cue from the post. Does it say they're venting - if so, I would probably offer empathetic support b/c poster is struggling. On the ED site, I'm more likely to challenge a person if the post reflects cognitive distortions, because seeing the ED thoughts can be very triggering to others. It also tends to set up a competitive environment of "who's the worst" that can really be harmful.

Here, I probably lean more towards supportive then challenging, unless several previous posts have indicated that he/she is heading down a bad/scary/dangerous path and to just provide empathy would be enabling.