I do think that what Grandma did is a BIG DEAL. Religion is a huge part of my life, and having someone decide that the way that I worship my God is not good enough, well, that would make me livid. In my family of origin, we were raised in X faith. As I grew, I decided that X faith didn't answer my questions and didn't make sense to me. I ended up converting to Y as an adult. When I told my mother that I was going to Y church, she was so upset she threw a shoe at me. It was a factor in the trouble that led to me and my mom not speaking for over 4 years. We are now on good terms, but if she ever did something like that you can bet your bottom dollar she wouldn't see my kids again.
Heck my sister is no longer worshiping in X faith. She doesn't go to any church. Niece has come to church with my family, but that has been cleared with Sister Every.Single.Time. I wouldn't bring Niece to church without her mother's expressed consent, much less have her secretly baptized.
At the end of the day, no matter your personal feelings about religion, Grandma went out of her way to do something that would upset the parents. She then lied and instigated a cover up. Grandma knew that what she had done was wrong, otherwise she would have simply told the parents what happened. That makes her untrustworthy in my book. As in, I wouldn't trust her.