Achievement hack

You don't need to be behind them to hack them. Also the wi-fi hack is a burn card. I think they actually hack other spectres.

Hacking is achieved by holding the reload button unless remapped.

For this achievement you need to use your phone to hack the achievements hahahahaha. Great minds think a like.

yup reported achievment hack aswell banned noob

Was your friend using a hacked profile? Normally when achievements start unlocking, it means that someone has hacked a profile to earn achievements which apply to everyone in the party. From my knowledge, there isn't a way to cancel achievements.

It looks like some of the achievements you got could have been legitimate...but I'm also assuming you didn't play 1999 rounds of GOW2 or you wouldn't be here...

You could absolutely be banned for this, despite it not being your "fault".

The best thing to do is contact Xbox live support, actually contact them via phone or support chat, and explain the situation. Don't expect an email or tweet to protect you.

It's an unfortunate consequence of people modding lobbies. They draw others in as well and really ruin people's Xbox Live experienced. Make sure you've filed complaints on the Gamertags from the lobby, and have your friends do the same.

Best of luck.