I don't want to go into huge detail online but if you had a colleague who you know is stressed - they have already taken some time off for stress, and have told you privately how they are feeling - and you can see they are still very stressed in work, is there anything you can do?
And if you were genuinely worried about their mental health, such as potentially suicidal comments, is it acceptable to talk to a line manager or HR, or would that be mixing personal/private information and work?
The complication is no other family members or close friends to try and contact instead (part of the reason for the said person's stress, I think)