The helmet must be removed for unconscious injured. If the person is fully able to respond you should leave the helmet on unless asked otherwise, but you must remove the helmet when the injured looses consciousness. Not taking the helmet of may have the injured die of blood or puke inhaled or just his tongue blocking respiration. So you're damned if you do and more damned if you don't.
Be careful when you remove the helmet, always remove glasses first, open the helmet and pull carefully straight "upwards". Keep the spine straight. Just before you completely remove the helmet use one hand to support the head, so it doesn't bump down. If there's a second person to help, one supports the neck and the other one pulls.
Just refresh your first aid class and do the best you can. It's in pretty much all cases better than no first aid.
Better yet, become a trained first responder yourself. And urge your family members and colleagues (and anyone you spend significant amounts of time with) to do so as well.
By the time a trained rescuer arrives, the biker is dead or has massive brain damage from oxygen loss. So whatever you do can only be better than doing nothing.
Be careful when you remove the helmet, always remove glasses first, open the helmet and pull carefully straight "upwards". Keep the spine straight. Just before you completely remove the helmet use one hand to support the head, so it doesn't bump down. If there's a second person to help, one supports the neck and the other one pulls.
Just refresh your first aid class and do the best you can. It's in pretty much all cases better than no first aid.