Yes of course. But meanwhile, there are sibling comments spreading anecdata about how their digestion makes them more/less sad and out of the corners might come the quacks with their depression-curing essence of beneficial stomach bugs or something.
I feel it important to point out that most to all such links as in the original article are in the very early stages of being investigated. And no personal or medical consequences should be drawn from that, and a lot of caution should be exercised.
Maybe understanding that the gut microflora and immune system state "may" impact mental states ("we haven't gathered sufficient data") is enough to cause people to think about their own diets and health outcomes.
Maybe we don't need to act like medical professionals are the all-seeing, all-knowing seers in their golden cathedrals. I went to school with a bunch of premeds and biochemists. I know exactly what's up. The science is slow and people get wrapped up in their own egos.
There's enough information right now for people to change their diets, health habits, etc. if they suspect it may make a difference. Maybe in the fullness of time we'll discover it had nothing at all to do with gut immune cells, but I'm not going to suggest that people sit around and wait for the Godot of biochem to tell them it's okay to start thinking about it. It wasn't that long ago that we weren't even suggesting making changes to accommodate depression.
One of the smartest people that I know deals with suicidal depression. From an outsider's perspective it's completely illogical. I would love for them to continue trying everything in their power to escape it, because it's crippling and ruins their quality of life. I'd rather that than just saying med up and deal with it.
I feel for you and your friend and hope they get well. But especially with mental illness, you only have so many tries at a therapy before the patient gives up on ever getting better. So I think the "see what sticks" and "you might as well try X" approach can be more dangerous than most people realize, because it eats up valuable patience. So I'd rather prioritize such suggestions, and maybe try the ones that are known to be working at least some of the time, first.
I feel it important to point out that most to all such links as in the original article are in the very early stages of being investigated. And no personal or medical consequences should be drawn from that, and a lot of caution should be exercised.