How did this resolve? If the grandmother truly feels bad and learned from her mistake, realizing that it was careless, the baby is probably safer in her care than ever, since she will be extra vigilant. I understand he worry and fear. My MIL hasn't injured my baby like THAT, but she has left him on the couch and walked away when he was too young to climb down, but old enough to crawl off, and he fell. She let him play in his great grandmother's glass China cabinet behind me while my great grandmother and my FIL said it was a bad idea. I was sitting at the table eating dinner, turned around, and gently said, "I don't think that's a good idea". My MIL said, "oh it's fine he's just playing and I'm watching him". My MIL is a bully. I was frustrated, since, as a mother, she should listen to me! But I turned around and picked at my dinner. Less than a minute later, the baby was screaming and crying, he had opened the cabinet, hit his eye and fallen. The MIL has let him play and touch the flat screen TV. When I said I didn't want him touching it because he could pull it over, she told me he can't pull it over, I'm being ridiculous. I know I'm not and I know babies die from pulling flat screen TVs over. I know that a small child can pull over much larger furniture. I stood firm on that one. She is highly educated, so should know better, but I feel it is about control. She has to be in control. When my baby was 1 week sort of a year old, my MIL babysat for us one night. When we got back, I noticed an open container of formula in the fridge. My baby hasn't had a drop of formula in his life and she knew that. She has babysat him before (though this was the first time without my FIL present too). She knows that we are anti-formula and in the past, I've told her she can give him a bottle of kefir watered down with coconut water, or juice. When I questioned her about the formula in the fridge, she claimed that she didn't know we didn't give him formula and that she assumed I meant formula when I told her she could give him a bottle. Even though we've NEVER had formula in our house (this was her house, her formula). She said he wouldn't take it anyway. He got fussy, she didn't see anything wrong with it. I was so mad. If she had said something like, "he was crying and I know you guys breastfeed, but I was desperate to make him happy so I didn't have to make you guys come home early, so I tried it" I would have been a little upset but ok with it. I could have respected that. But to lie and backpedal, and blame it on me for not being perfectly clear made me mad. Because I KNOW she knew we didn't want him to have formula. We had been through this discussion before!
So, while my baby hasn't been seriously injured, every minor injury involves my MIL and my MIL always dismisses my concerns. I feel I wouldn't be wrong if I didn't let her be alone with him anymore. Not because of accidents, but because she doesn't respect our wishes as parents. Luckily my FIL is usually there and he is responsible and respectful. I would understand your GF not wanting to leave the baby alone with your mom, but I don't think it's right to not let her see her grand baby. If your mother is normally responsible, and is respectful of your rules, and is willing to make any changes you and your GF see fit to make it safer, I don't think she should be punished like this. In fact, she will probably be hyper vigilant after an incident like that! I think the three of you should have talked, and spent some time all together with the baby. Grandparents are important for kids! I hope your GF recovered from her shock and allowed your mom to be part of the baby's life. Accidents do happen. When I was a toddler I pulled a pot of boiling broccoli and spilled it down my chest. They are surprised there aren't scars. But just imagine how guilty they felt, knowing it was their fault!
I hope your family healed from this incident!