Without a massive boost in senility, a lobotomy or way too many psychoactive drugs, isn't it kind of hard for one to become a "former genius?"
Is "genius" some kind of IQ score that is measured abstractly? Can one be a "genius" behind bars or homeless? Or is "genius" measured by actions, as in inventing something like a filesystem? If the latter, does one piercing insight net you the genius title for life, or must one keep it up?
I can see how one definition of the word "genius" might be the realization of creative and intellectual potential, and one could consider this type of genius a temporary title that must be "renewed" through ongoing and active creation and intellectual activity.
This particular definition seems to be in harmony with widespread HN values, namely that it is not enough to have ideas, one must also manifest them to earn full admiration. In which case, Mt. Reiser might well be a former genius for the purposes of ongoing realization of creative and intellectual potential.
Since we're talking about a headline instead of a clinical diagnosis however, I'd suggest the author was using that phrasing as a more concise way of communicating "formerly known as a software genius, then famous as a convicted murderer, and now working on a shtick as a mad scientist."
Reiser's behaviour has always been bizarre by "normal" standards. It's only recently that it first tipped over (murder) and then was put under pressure and showcased to the world (investigation and trial), but there is little indication that he would he behaved more "normal" if a similar situation happened earlier in his life.
genius you are, genius you stay, the dash doesn't wash up.