I have depression myself. Have done since I was 16 or so. When I'm well, I good, but it can strike at any time & my mood plummets. There can be a trigger like a harsh word from someone or stress.
My symptoms are.....
1: Isolating. When I'm unwell I don't answer the door or phone & don't leave the house unless I urgently need food.
2: I can't sleep. I have to resort to sleeping pills. If I didn't, I'd get more unwell & that's when thoughts of suicide creep in.
3: I can't be around anyone as I believe they don't like me & I just want to be alone.
4: I get very irritable & can lash out at insignificant things.
I denied I had depression for a very long time. I believed I was too strong & intelligent to have any sort of mental illness &, of course, didn't want one. So I suffered for 10 years until an attempted suicide got me into the mental health system.
Men are harder on themselves when it comes to mental health issues. If your friend broaches the subject with her partner, he may get defensive & say he's fine - just tired. Or he may get very angry at the suggestion of it - how dare she intimate he might be mentally unwell, that's for sissys!
So if he does have a mental illness & won't accept it, then accept help for it - your friend has a rather hard time ahead. But she mustn't blame herself or lose herself. She will have to look after herself & stay healthy.
But it might be he's on drugs or he wants to end the relationship or anything else.
Anyway - good luck to your friend & her partner.