She knew he was just trying to help. I think she didn't appreciate having the crying child brought to her attention which would have interrupted her conversation she was having with her friends.
The insinuation was, "stay out of my business or I'm going to tell a lie that could ruin you". He was clearly not bothering the child, anyone could see that, she could saw it herself. Whatever her game was, it was completely deliberate.
Since the child wasn't actually in any real danger, we chose to simply remove ourselves from that situation and not involves ourselves with a crazy person. Unfortunately being a shitty parent isn't illegal.
To steel man the mom, I raise my own children with a “rub some dirt on it” mentality for non serious injuries, minor scrapes and such.
Depending on the degree of injury, it could really be nothing and the mother is parenting to her standards with her child.
Personally, I’ve had to routinely counsel my own mother who would rush in with the overblown “OH MY GOD ARE YOU OK?! OH NO YOU SCRAPED YOUR KNEE!” Which in turn rile up my kids and cause a bigger scene overall. Typically my interaction with my kid is “Let me see… oh that no no big deal, you’re fine, get back out there,” and they calmly accept that it’s a minor injury, no big deal, and they keep playing. No screaming fits needed.