Wish I had not overheard - none of my business but feel sorry for parent

A lady came to visit with her baby. She had been back at work a few months and was concerned about her childminder, and looking to find a replacement. I felt we got on well and I really empathised with her, and felt it was completely understandable that in the end she decided to choose a nanny share instead of another childminder.
She told me her childminder helped run a playgroup whilst minding three under 5s, and that her baby had escaped crawling out the door and been rescued as the childminder was busy doing other things. She had been told about this by a friend who witnessed it. When she asked the childminder if it was true, the childminder had got angry and defensive. The childminder then became unhappy as the mum wanted to reduce her hours at work. The mum felt their relationship had broken down. On the day she came to me the baby had three large scratches on his face which she said the childminder just said she didn't know how they happened.
I had my suspicions who the childminder was - someone I know by sight only. Our paths never cross usually. Anyhow today she appeared at a playgroup I frequent along with another lady who to my surprise was looking after this baby and another child. They then proceeded to have a moan about his mum for at least half an hour - generally bad- mouthing her and putting her down. It seemed they were friends and that the childminder was very bitter about this woman.
Anyhow I feel bad that she may not know her new nanny is friends with her old childminder. And that although the baby was not neglected these childcarers both seemed to have an attitude that they knew best and that parents are too fussy and safety concerned. I know it is none of my business but I feel quite angry at their attitude and the fact that they give other childminders a bad name. I have had some parents who are as right pain but I don't share it out loud in a public place.
Also is it OK to help run a playgroup whilst minding?

I think in principle it is okay to help run a playgroup whilst minding. Hopefully everybody chips in to lend a hand and there isn't too much for any one person to do. I say in principle because it depends if they are able to cope with watching over their mindees as well as doing whatever it is they need to do, and if the babies are escaping, that would suggest they are not being very effective at their job, but you don't know if this is a one-off lapse or a general rule, or how much the incident has been exaggerated in the retelling.

It is definitely bad form to criticise parents loudly and in public, and will get her/them a reputation for being indiscrete and frankly rather unpleasant. But in your shoes I would not get involved - it could just end up looking like sour grapes and the minder and her nanny friend could end up retaliating by spreading gossip about you or making your life difficult in other ways.

Many CMs run playgroups whilst childminding. It's just that some are better at delegating jobs than others. But still that CM should not bad mouth the mum :(

Oh dear, I can see why you are concerned! However, I would think the mum will soon get wind if there's issues with the nanny so if I were you I would not involved )unless of course there is a safeguarding issue)!!

It really, really saddens me when carers bad mouth parents in front of the child - I know you said the mindee was only a baby but even so. I would hate for my daughter to be aware that someone was talking horribly about me. Like others have said, unless there is a safeguarding issue you shouldn't get involved. Shame for the mum that she didn't go with you. x

Thanks everyone for your spot on advice. I guess I just feel sad as they were such a genuinely lovely mum and baby and I really wanted them to find the right person. I am lucky because in my area there are so many mums looking for childcare so I can pick and choose work. But I really see the downside of this for parents.

Yep I think d steer clear of it, unless you have serious safeguarding concerns.

Sadly some are unprofessional. I'd never bad mouth a parent although having said that I do ask for advice from some trusted cm's I know,