The only ways I would allow someone with a gun out to stop the melee weapon is either if they have the dodge skill, or if they are currently fighting him back in melee with the gun as an improvised weapon. Guns in a lot of ways have advantages over melee weapons and are usually stronger, although this is one of the big things that can make melee weapons good against people with guns out. After the charge attack, if they want to start swinging back with their gun and parrying on their next turn, they can go ahead, although if they just shot their gun at someone, I would consider them in firing mode and not melee mode, therefore not allowing the parry.