[foldr]

I think the most charitable interpretation is that they're having some kind of mental health issue. You could tell them that if they're currently unable to work for unrevealed personal reasons then they have to be considered on sick leave. That means that after some reasonable grace period they have to supply documentation. At that point, you'll either know more about what the underling issue is, or have very solid grounds for firing them.

I think firing them now is also a reasonable decision to make (but perhaps a little on the harsh side, depending on the time frame).

[connordoner]

Thanks! I think the whole sick leave thing is probably going to be our best choice.