He's talking to the video game, I can tell from the sounds, so at least it doesn't escalate into an argument. Still, the kid is nine. A number of thoughts run through my head.
1. The kind of kid who would never say a bad word is the kind of kid who would not be interested in being friends with my son. He's got issues; it's a fact. His friends are all a little rough around the edges. (But boy are they forgive-and-forget kids, which is also super important.)
2. Son's friend has a mom who cusses pretty regularly and enthusiastically. She also lets him play games that are rated M. I'm thinking "Your son said the f-word" is not news that is going to surprise her, let alone upset her.
3. My usual middle teacher response would be something along the lines of, "Hey--language." Code for, "I heard that and I don't think it's appropriate. I'm not going to get into a power struggle about it, but I want you to know that I heard you and will call you on it if I hear it again."
4. But my son is right there, and he has this weird thing with "bad" language. He likes to test it out, awkwardly and obviously, giggling to himself. If I don't react, it disappears. If I do react, it becomes this enormous temptation to keep saying it. Not in context, just by itself, slipping past the hands he's clapped over his mouth to try to keep it in.
So I let it go, and then continue to second guess myself all afternoon.