I would say no. I remember my niece being ill and home from school and my sister having something important on at work she was in a panic worrying about her until I could get over and pick her up (she lives about an hour away) Being ill and alone isnt very nice especially for a child.
As far as I know, the visit to friend is just to catch up and nothing of importance.
The child could call his mum if he needed her, but i'd say it would take her about 20 minutes to get back home.
If he'd not been sick today and was slightly improved, and if the mum had had to go to work, and not to socialise then I think i'd feel more ok with it, but as it is I do think she is wrong to leave him.
I might, if the child was OK with it and could phone if they felt ill or lonely. I was a very happy latchkey child, it seems normal to me, and now I do so with my dc. they love it and beg for more time home alone. lots of children don't see it as a negative thing.
No! DD is ill today and I left her for 20 mins to take DS to school, plus she wrapped up and sat in the car with her mobile whilst I did a very quick supermarket run. It's pretty miserable to feel ill, and to be deserted on top of that (something my dear mum did a lot) is just heartless, unless it's an emergency.