Kids making fun of retarded people?

Yeah so i was in a game of Black Ops earlier and there seemed to be this one kid on my team who had some sort of speech impediment and i think he probably had a mental disability do. Well there were these other 2 people on my team who were telling him to say his own gamertag which was "Blackbird" with some numbers after it and he would pronounce it "BwackBuwd" and then the kids would start laughing. They also told him to sing his favorite song so he started singing "We all live in a yellow submarine" by the Beatles and he pronounced everything wrong and the kids started laughing hysterically. To be honest, I laughed a bit even though it's kind of wrong but I didn't contribute to what they were doing at all. At first i thought it was pretty messed up that they were doing this, but then again they weren't really directly making fun of him. They weren't straight up calling him a retard and stuff like that and the kid didn't even seem to care about what they were saying. Also, he sounded like he was about 12 or 13 years old so he shouldn't of even been playing the game in the first place and at least don't use a god damn headset if you got problems like that!! Whatever, it's probably the parent's fault. Just out of curiosity how messed up do you think this is from a scale of 1 - 10??