I can see that the grandmother could have been upset or offended by the whole thing, although to be honest, if a child made up a similar story about me and a parent asked me about it, I wouldn't be offended because safety trumps etiquette. It's what a good parent does. I would be worried, sure, about the fact that the child is making up weird stuff, but I would not hold it against the parent because it's the right thing to do.
That said, let's suppose grandmother was offended and bf was upset/protective over this. He needs to talk to the parents, then, not the five year old. You don't solve things by talking to a kid that young. If grandmother wanted to bring it up, then fine, but it didn't involve the bf and it was clearly a touchy subject that he knew SS felt bad about. I don't see any reason for him to bring it up except to scold SS, which isn't his place to do. Grandmother's to some extent, sure, but not his. It doesn't involve him. He should have talked to the parents in this situation if he felt a need to. Maybe, and this is giving him a big benefit of the doubt, he hugely overestimated the maturity level of five year olds.
I wouldn't let SS have any more unsupervised time with grandmother. She covered up a drowning story for a week(!!!), and only told the truth because she didn't want him to tattle on her? What planet is this okay on? Her reason for coming out with the truth hardly suggests that she'll do the right thing again in the future. She wasn't sorry she did the wrong thing that almost lead to a child's death. She was sorry that she might have gotten caught. Watching unsuitable television with a child in the room is far less serious, but still not acceptable. What if it was hardcore pornography? "Oh, he's just playing on the iPad, he wasn't paying attention," obviously wouldn't cut it. It's an excuse that just doesn't work. If a child is in the same room as a television, a person, or anything else, then they will learn from it to some extent. Clearly he was paying attention enough to make up a story which was horrifying enough to need to be checked out. Maybe grandmother has gotten better since she almost let her grandson drown, but she still has shown herself to not be quite responsible enough to be watching children. The fact that she made excuses about the TV show implies that she knows it was the wrong thing to do, but did it anyway.