Hi
Thardey
I'm certainly no expert on these matters (having had to talk someone away from the brink only once) but I can say from experience that the 'severing of ties' is common side-effect of depression.
It can come from both sides of a friendship and can be passive or active.
What I mean by 'passive' is that is that friendships can be allowed to 'die' by simply not 'feeding' them any more (I basically went to work and hid in my flat each day for a couple of years, which pretty much killed all my friendships). The 'active' approach is where an actual choice to end the friendship is made and sometimes vocalised (sometimes bluntly

).
Regardless, that 'withdrawal from the world' is not a good sign and, tho' your erstwhile friend might not appreciate it now, it may be worthwhile just to check on him every now and again. I had one old friend that did that for me when I was in the depths and in the end that slender life-line back to the 'outside' did the trick.
As I disclaimed tho', I'm no expert, I've just lived through my 'version' of the Black Pit, so I could be way off. I don't want to speak for someone or put them on the spot but it might be worth having a word with
myusername as he's worked in the field and may have some experience that will be helpful.