There but for the Grace of God go I.......
I admit I've taken risks with the children when they were babies. We all have. I know mums who have changed babies on tables, not realising they could roll until they rolled off the table when their back was turned.
A friend of mine didn't realise her dd could reach the window latch from her cot, climb out and open the window. She didn't realise until she saw her dd fall from a first floor window onto the concrete below. Luckily her dd was ok apart from a large lump on her head, but when she was sobbing myself and the other mums from the baby group all comforted her, we never told her how stupid she was. Because we all knew that none of us were perfect and we all made mistakes.
If your girlfriend had made this mistake, would you let her look after your son again? Of course you would.
It's so easy to say "I would never do this" but the truth is that I don't know a single parent who has never made a mistake.
We all know that children should never go near pans or kettles as they have a tendency to grab, but we sometimes, erroneously, think that are in control of the situation, that they are far enough out of reach.
It was a mistake with unfortunate consequences, but it's not as if your mother has a habit of doing this? She didn't neglect her grandson, she didn't deliberately put him in harms way, she made a gross error of judgement that she has been truly punished for and will forever remember when she sees the scar on his leg. Just as my friend will forever be reminded of the accident her dd had with the little scar she has on her forehead, yet no-one threatened to take her dd away from her or suggested she was an irresponsible parent. Her love for her dd was apparent. She made an error of judgement, as your mother did.
I agree that your girlfriend, for now, is still acting out of shock and yes of course she blames your mum for the scarring your son now has. In her eyes she would never ever have done this and you can't tell her any different.
She will calm down though and the shock will subside.
Has your mother tried all she can to make it up to her? Have they spoken at length? It might be an idea to have your mother round to your house perhaps with her parents too so that there are other people around to diffuse the situation and offer more reasonable responses. Your mother can then stress how sorry she is and it would be helpful if she agreed to some of your girlfriend's conditions, so your gf can see that your mother is genuinely trying to make it up to her.
She can also go on a First Aid course for children, that might put your gf's mind at rest that your mother is now fully aware of all the dangers that children can face and knows what to do in case of an emergency. Would she sign up to such a course? Could you find out about that for her?
Give it time. Be sympathetic to both parties. Your gf will come round eventually. It's a heck of a shock and to be honest, when something like that happens to one of your children, it's only natural to want to blame someone. You have to direct your anger somewhere. But trust me, that anger does pass and you do start to see things more reasonably.
Good luck with it all.