Well, true, she may have a health condition that I don't know about. And of course, so she is entitled to claim for benefits and be paid accordingly.
But, I also have an invisible health problem with a past head injury and post concussion syndrome that i had nearly 9 years ago, and a subsequent iatrogenic injury from a prescribed off label antipsychotic after the head injury (which is a neurological involuntary movement disorder that is like Tourette's and Parkinson's disease combined).
I'm not entitled to any benefits because of this, and we earn just over the threshold to be entitled to any UC. So, basically I need to work in spite of my brain not working properly and it giving me cognitive problems. So, in some ways, I find it hard to keep going but I do. It's not clear cut, as you say, but often as you also point out, people need to keep on working regardless.
If my DH's friend had a health problem that stops her from working properly, fair enough. My assumption was that she decided that she'd be better off on benefits than working (and she's actually said this to my DH) so I'm basing my opinion on that.