If his round continues, his score will count to the average. The XPs will only be for a single round, lower than in company. It's a sad situation for him to take the time to play multiplayer with you and end up as a single.
You will fail to get the XPs, delaying your level-up proceedings. Your intention to protect your average(?) will be in vain because a bad round does not affect the conditions necessary to tier up. Your gain is zero.
And, if the game crashes due to your quitting, your companion ends up with empty hands like you, wasted time, no score, no XPs. It's a lose-lose situation.
Thus, forfeiting or quitting without a good reason is poor sportsmanship.
Make your decision.