so why approach this as if he’s definitely mentally ill and that’s why he’s doing this?
Yes, there’s always a chance that people posting about their partners mental health are the victim of an abusive or lazy partner. If you look back through the few posts made by the OP you can see she’s now worried about him and there’s no suggestion of signs of abuse or laziness despite previous posters suggesting as much. Also he’s displaying my early symptoms!
If she has doubts about her partner and thinks he’s abusive or lazy then of course that would change my view. But until then, if she’s not anything other than worried about him so I’ll believe her and accept her judgement at face value 
A man who calls in sick, for a second time, says he feels depressed (which a lot of people say as a word for being down or sad!) but has no other indicators of having depression, hasn’t seen a GP about it or being diagnosed
As you said, depression manifests itself in many ways and the OP’s short posts have described my indicators, indicators you’d miss if you knew me as you don’t recognise them as such!
My DH and I are now both diagnosed but both of us had to practically fall apart and been told by the other to go to a doctor. We didn’t go when the undiagnosed signs were small because we didn’t recognise them for what they were or thought we could handle it.
I’m one who needs to organise and simplify my life in the run up to feeling more overwhelmed. If those coping strategies don’t work then things snowball and I get the symptoms that described you. I’m a long way down my path of dealing with depression and haven’t been depressed or on medication for over 20 years thanks to counselling which identified triggers and coping strategies but I still have issues with food. The indicators that I’m struggling are that the house is always tidier and I catch up on doing the books plus bulk buy in shopping and batch cook. I don’t go out as much and avoid contact with others. My coping strategies work well so I never need to get to the extreme symptoms stage. It was the same when I was undiagnosed but I didn’t recognise them for what they were.
Both my DH and I had to exhibit far more extreme symptoms before we went to the doctor, and even then only after we had be ‘told’ to by the other. I don’t think it’s common to self refer to a doctor until you get to the stage where you aren’t coping.