The girl should tell some of her regular clients about this guy. I have dealt with a few a-holes for a few girls myself. Don't involve the police, they just make things much worse and remember they always let them out on bail, and he has her I.D !!!!
Drug dealer with a gun? .. Only real low life punks have guns. You should report the gun threat. you can give the address and name anonymously !!. At least do that.