Yes, that was me! I don't have any grandchildren yet but I well remember those early years and there were no grandparents to help out.
There have been a few episodes of Woman's Hour lately featuring young women having a hard time being mothers. I think the difference now is that they are allowed to admit it. I remember it being completely disallowed.
If she doesn't have any friends nearby, could she find some online, perhaps on Mumsnet. Aren't there any toddler groups where she could at least meet other young mothers. Probably the only cure, as such, is time and a return to work. It probably really helps to have you to talk to and help even if she seems ungrateful and doesn't want you to tell her it'll get better. She can't hear the latter yet, she's too buried in it all. Sorry if this doesn't help much, I do remember those early days and how awful they were.