What can you do? A percieved "helper", a policeman, opens fire on your group.

Beat feet. Get cover/concealment between you and the shooter. Continue to move, tough to hit a running target, and move in a way it keeps you off the central line of fire of the shooter - try and stay in back of him or to the sides. I wouldn't hide and remain static if there is a chance he will find you/get to you. Lock doors behind you if in a building, slow him down. Never go up if you can help it, boxes you in. Pull a fire alarm if in a building. If low enough, 1st or second floor, break a window and get out.

Not going to get into the discussion about police being minutes away at best - not their fault they are spread thin. Nor am I going to talk about improvised weapons - the dude has a gun, he can kill you in an instant. Don't give him that instant.