in my opinion, potty training is really psychological and has a lot to do with a child developing a sense of will, listening to their own body signals and being able to tune into knowing "when." that's a child's individual process. my son is potty training right now and our first step is showing him how to sit on the potty right after wakes up (this is when he usually has a bowel movement) and reward him with 2 m & m's...i haven't yet taken the next step with him because i don't think he's totally ready yet. most kids who are ready to potty train show a bit of a willingness when you try and explain using the potty to them.
my husband sometimes gets upset at my son for pooping in his diaper because it really is a royal mess, but i explain to my husband that my son should NEVER be made to feel ashamed of pooping in this diaper because really, if my son knew better, i know he wouldn't do it, and my son is just not there yet. plus pooping is a very natural and private/personal process and it should be treated as such.
an example and experience i'd like to share with you is this. right before my daughter turned 3 years old, i tried to put her on the potty because her poopy diapers were coming out of the dipe and making huge messes. i tried for about a week putting her on the potty, scolding her, etc, and i got SOOOOOO frustrated, but at the same time something didn't feel right. i felt instinctually that potty training shouldn't feel like a struggle, that if it did, i probably wasn't taking the right approach. i had studied and read books on the subconscious and one of the concepts is that the mind is the most receptive right before bedtime and right before waking up in the morning. so soon after she was asleep, and also before she woke up in the morning, i would say, "everytime you feel like going pee or poo, you go straight to to the potty. you like to use the potty. it feels good." (and other positive suggestions)....not long after that i was making lunch one day and i hear "mommy, i'm using the potty!" i found my daughter sitting on the toilet by herself. just like that, my daughter started using the toilet, on her own, when she was ready, with NO struggles, and we never had accidents. it blew my mind how easy it was.
so anyway, i guess i think potty training is a readiness thing, and all kids are different as are their personalities and paths of development.
you are a good grandmother for taking notice of this situation with your granddaughter. you could also just encourage your granddaugther and tell her how proud of her you are and that you know she is smart enough to make her own choices and act upon her instincts when she is ready no matter what anyone says.
hope that helps!