Do I thinj what the Op's friend did is ok? Nope, dont leave a baby home while you nip to the shops.
Do I think, in the absence of any other alarming behaviour, that the mother doing this once warrants being reported? No.
Would I tell them that I thibk its is not a good idea, potentially dangerous and worrying? Yup.
If it happened more than onve or became a regular occurrence or there was other alarming behaviour to go along with this, then I'd consider reporting. But not for an isolated incident.