He probably means an option to make bullets that era shot at your teammate, not collide with him. They'd go straight through him in that case.
If that's what it is: F11.
yes, the player could activate/to disactivate this option.
not collide= good in closed places, you can make right targets without making right its friend(barret,law),
collide = good to give impulse to the allies(law,m79,minigun,nades..)]
it is enough to pressure a button to activate/to disactivate collision in game.
sorry,my ingles is bad =/