Ok, so there'll probably be a deletion on this shortly for a PBP, but in the meantime....
Firstly, there's no minimum age to leave a child home alone, so nothing illegal has happened here. HOWEVER, the consequences stack up thick and fast if something goes wrong, and your friend should be aware of that.
Also, going to the pub doesn't automatically equal paraletic and unable to cope/be responsible. If it DOES, again, the consequences stack up thick and fast.
Where I live, SS are VERY busy, very stretched. Understaffed and underfunded. I doubt they'd take this on. That doesn't mean it isn't worth reporting.
I report regularly, KNOWING they won't take up my referrals. But I also know the drip-feed of my referrals helps build up a picture and when a family DOES come within their radar, they often escalate if they have all my reports of my concerns.
A 1 yr old is VERY young. I deal with primary aged children (mostly). They MIGHT sleep through. But... if they DID wake up. I don't think SS would be happy with this scenario.