I can't believe people are trying to diagnose her with puerperal psychosis, every time that a certain missing childs name comes up on these threads there is always a horde of people claiming it perfectly normal to leave babies and children unattended while going out to socialise, the chances are she's just one of those ridiculously selfish idiots, there is a disturbing number of them if mumsnet is anything to go by, it seems the done thing in certain circles.
It's disturbing how detached some people are to their babies and their needs, most are not mentally ill, just too self absorbed to care about anybody but themselves. She needs a kick up the arse, to understand how utterly selfish and neglectful she is being, how unacceptable her behaviour is and that it won't be tolerated. The threat of social workers swooping in to take the kid is enough for some parents to cop themselves on.
I hate how every time someone behaves abominably, the threads get taken over by people claiming, with no evidence, that it's mental illness (or aspergers, sn etc). Most crimes, including neglectful, shit parenting are committed by perfectly 'sane', neurotypical brained people, it's ridiculous to try to excuse the actions of self absorbed dickheads by claiming that they must be victims in some way, they must be crazy/sick/incapable....
Call social services, if she does it once, she'll do it again and again until something happens. If she genuinely has mental health problems they'll identify that and will help her, in the most likely event that she's a selfish tit, they'll scare her into being more careful with her child.