As someone who suffers a lot with depression, it is quite possible to be depressed and for no one close to you to notice. We can be damn good at hiding it, especially with practice, especially with people we know well.
Some time ago I was living with my partner and she had no clue I had been depressed for months. Then we went out with a couple we hadn't seen for a little while, and afterwards the girl took my partner aside and asked if I was OK.
Taren obviously knows him best, and I agree that if she says Aaron wouldn't have committed suicide if it weren't for the case then clearly that is true. However, I think she is dismissing the impact of depression, or at least dismissing the possibility, to readily. You could turn around and suggest that if he hadn't been in depression (if indeed he was) then the case may have not driven him to suicide.
Depression is a horrible, and terrible thing. Those close to depressed individuals certainly have a lot of insight, but it is still nearly impossible to communicate what it is like even to them.
EDIT: I'm cautious to add this, because it is "one guy on the internet", but reading Taren's post again some of the things she describes strike me as telltale signs of secreted depression. Disappearing on a whim for a hike, for example, is a favourite of mine.
EDIT2: It is also worth noting she describes one common set of symptoms of depression (low activity, disinterest, etc.). However, I for one don't often get them (for example) but I do become deeply involved in subjects for periods of time before moving on. Similar to how she describes. This is a very very common symptom of depression and if you see a friend become obsessed with a string of new things take a second to think about how they might be.