I would never leave my 2 week old either. In fact on another thread I mentioned I don't leave them for the first year. But I wouldn't judge someone else for "needing" to. And I truly wouldn't judge, not just say so then proceed to do so.
I would worry if she starts to exhibit a pattern of such behavior but not the first time or even every time she went out. I say this as someone who has never been the "go out for drinks with the girls" type. It's not my thing and it may or may not be moral depending on how much they drink, where they go or what entertainment they have, but aside from that I do understand some people need a break. I hesitate to think it has anything to do with not loving her baby.
It's been 9 months so perhaps you forget how exhausting the nine months of pregnancy, childbirth and the first few weeks with a newborn are. I don't blame any mother looking for a bit of recreation once in a while. In fact it's essential to her sanity sometimes. The first advice against shaken baby syndrome is to step away when you are overwhelmed. Not to say that's what's happening here but I am making the point that stepping away is not intrinsically evil. It can be a good. And we don't know what her circuмstances are. I have a lot of patience and calm with babies but I know it's more than most. In other words I am grateful for it but realize it's above average and don't expect everyone else to be like me in that respect. I try not to look down on those who don't have the same level of patience unless they show bad will in their impatience.
There's also personalities to take into consideration. I am introverted. The most refreshing thing for me is peace and quiet. DH took everyone but the baby out of the house for a couple hours today. That was so refreshing for me! The quiet ... Ahh... That's how he " spoils" me now and then. But people who are extroverted are refreshed by going out and noise and by being with other people. As much as I can't fathom it ( thats the definition of hell for me!) I do understand it intellectually. If she is extroverted it makes perfect sense that at the exact time when some of us think there would be nothing in the world better than to stay home with our babies, she is seeking refreshment outside. If that's the case she isn't alone in that and is behaving quite in line with her extroversion.
Of course she may be a terrible mother and all my points are for naught but I hope it gives some food for thought to help you perhaps be more well rounded in your assessment. Unless a pattern of behavior arises it's very premature to be calling her ridiculous or assuming she is a bad mother (implied in all the mothers of today talk) Especially with what little we have to go on.