punishment?

SKI1119
Ok I know this is not my kid but it seems so wrong-
My boyfriend has a 6 year old we've been together for 5 1/2 years so I've been in his sons life since he was small.
We get him on the weekends but there is not a court agreement set up.
He is a great kid and very sweet 
He got in trouble at school  for stabbing someone with a pencil he said they were playing zombies on the playground n he said it made a little mark and he felt bad and said sorry right away. I know he wouldn't intentionally do it. 
Well for punishment his mom tried to ground him from our family Christmas party today- well he got to go but he couldn't play with anything he got . And he says at home he isn't allowed to watch tv and missed out on their family trip to look st Christmas lights.
She also grounded him from our house Friday night but he got to stay at his nanas house and got to play on iPad- 
So is this fair??