I disagree - I thought there were a few worrying points in the OP that would concern me if it was a friend of mine.
Firstly, that she 'decided' to be polyamorous two months after giving birth and went quickly into having sex with someone she met online without her partner and is now 'head over heels'. I'd worry that:
A) she is particularly vulnerable in the first weeks after having her first baby and out-of-character behaviour could suggest PND or other post-partum mental issues.
B) from that description, she wasn't especially safe - I've read advice on here to people contemplating swinging and it's always been to go slowly, talk it through thoroughly with your partner, start out going to a safe environment like a club, don't go straight to one-on-one sex with a stranger etc.
C) the 'head over heels' part sounds like there is a strong emotional connection and afaik that's not really the idea of swinging so I'd be worried about the strain placed on her marriage, which already has the stress of a brand new baby.
I honestly don't think this is about judging sexual morality or insisting that mothers can't be sexual (if the genders were reversed in the OP, I bet the husband would have come in for tons more contemplation). But this, even without all the additional information added later, would ring alarm bells for me. The dripfed info obviously sets off nuclear sirens, but the original info is concerning.
If it had read 'my friend and her husband have always been interested in swinging and since their baby was two months old they've been chatting online with like-minded people on swinging sites. They've attended clubs and had sex with people there and have had good quality childcare for the baby, who they clearly love and cherish. AIBU to think she's a slag?' - well, then those of us who have recently given birth would squint disbelievingly through our bleary, sleep-deprived eyes and mutter 'Christ alive, rather her than me but good luck to her' whilst we wincingly try to find a comfy way to sit down.
But that wasn't the scenario in the OP and I think it would be an irresponsible friend who didn't express a bit of concern and worry. I think some of the responses have been overly harsh and not helpful.