Never do it! The bullet would have flown unsuspended in the chamber for a bit untill it hit the rifling. I imagine that if the bullet had left the case a little crooked, it could have smacked the side of the chamber some, but probably wouldn't be enough to do any real damage.

Your friend should be thankful you don't shoot a .308, That would have done some real damage, definately to his gun, and probably to him as well.

I can't understand how someone would not notice the difference between a 30-06 length case, and a .308 case. Always make sure that you are putting the right ammo in your gun! there is upwards of 50,000 PSI of force when you pull the trigger!