The best thing is just to ignore completely. If you engage at all, you risk worsening their situation by wasting their time, while if you do engage you might be saving someone else at their expense…. It’s lose/lose to engage in any way.
I’ve settled on just saying, hey, I’m sorry that they are in the situation they are in, and I hope they find the strength to make it out someday. That makes it clear that there is nothing to be gained while offering a tiny bit of hope and empathy, which I’m sure is in short supply.