My cousnnwho is new to the game played a firefight arcade game on my account getting a 29 kills and 9 deaths will i gwt banned because he did so badly.
I am pretty sure you are good to go. 
If i were you, make him play customs FF until he averages at least 70 plus kills. For all 343i knows it looks boosted. bungie even stated the same thing as well if you were on the bungie forums. I wouldn’t continue this progress mm ff until the progress has been made in customs. Hope this helps!!
> I am pretty sure you are good to go.
i hope you are right snicker doodle, I’ve seen a plenty of accounts get pentalized for not having (enough kills)!! 
> My cousnnwho is new to the game played a firefight arcade game on my account getting a 29 kills and 9 deaths will i gwt banned because he did so badly.
I looked at your game history and found no such game anytime in the recent past. It’s unlikely someone would get banned for only getting 29 kills. Perhaps he was AFK for part of the game.
If this really happened, consider it a life lesson. It’s your account and you’re responsible for anything done under it.