I'm not really sure what you're asking here. Obviously, no one here can assure you that she won't get in trouble for her past criminal activity. Obviously, however, not committing similar crimes in the future would be a smart thing to do.
By the way, most people would be embarrassed about something like this. That your grandmother actually bragged to you about it suggests to me that this is a more significant problem for her and that her claim that she "only" did it once before may be a lie.
Given the small prices involved this time, it's probably best not to call attention to it, and it certainly wouldn't be a good idea to compound the already committed crime with a lie.

