My friend is so sensitive because my younger cousin roasted her. How can I get her to stop being a coward?

So yesterday I was looking after my 10 years old cousin (let's call her Natalie) and she, by best friend and I were playing a video game. Natalie was roasting both of us but I got her with a comeback but of course I was careful what I was saying because she is just a kid. My best friend on the other hand took it very offensive. Natalie was roasting the HELL outta her. I was like, "Are you going to take that from a 5th grader?" She tried to get a comeback but Natalie got her again. I was like, "Okay Natalie that's enough. You're taking it too far." Natalie said, "It's just a game. It isn't my fault your best friend can't take a joke. I would have read about her before I play with her."
My best friend was crying so hard. I told her, "Oh buck up. You shouldn't let a 10 year old offend you like that. It's just a joke. Plus she did take it too far but instead of running off crying like a coward, you need to stand up for yourself. Kids aren't going to respect you if you don't have thick skin." I told Natalie she needs to be careful what she is saying.