I know someone like this - I used to work with her. She had a serious mental illness & became a bit of a nightmare to work with. Partly because she was in denial about the nature of her disease (schizophrenia) and said it was really migraines. And partly because the illness is a really difficult one, and I can imagine there were days when she just couldn't face leaving the house. We all tried to understand, but 5 years of covering for her, doing her work, trying to cover for her with her clients - because of course we couldn't say why she really hadn't done X.
I left that job, but I know that our manager was just starting to instigate a capacity enquiry - she couldn't satisfty the requirements of a full-time job, even with reasonable adjustments. And we were starting to get sick of doing her work & patching up the mess she left with clients.
She could never be relied on to undertake any specific speaking engagement - she's say she had a migraine. But she would still keep volunteering - we understaood that her illness meant things were difficult.
But what was increasingly impossible was her denial. So she'd keep volunteering to do stuff, then dip out at the last minute, leaving us to cover. In the end, one person was alsways delegated to shadow her workload so they could take over.
I found I couldn't stand it, and left. We had been friends, but I had to just back away as I just got so angry with her - not the illness, but her denial that she had the illness. If she'd faced up to it, we could have supported her.