The church-sponsored, non-scary "trunk-or-treats" that people have mentioned are very good alternatives.
Also, the National Air and Space Museum's Udvar-Hazy Center (the branch near Dulles Airport, not in D.C.) holds an "Air and Scare" trick-or-treat party inside the museum on Sat., Oct. 25, starting at 4 p.m. I haven't been and don't know how scary it is but probably it's not too scary--I hear there are adults who go around in "Star Wars" costumes and the kids get to get candy at different stations. Great if a kid is into airplanes, of which there are a LOT at that site. The event is free, I'm told, but you may have to pay for parking, which can be stiff there.
The "Boo at the Zoo" gets rave reviews as someone mentioned but be aware that it requires paid tickets -- you can't just turn up -- and those tickets often sell out well in advance, so check their web site or call now if you're interested.
Another word of advice to anyone whose kid is scared easily -- I have been distressed to see craft stores (yes, Michaels, I mean you) displaying realistic, life-sized "murdered bride" figures of a wedding-dress-clad, headless dummy holding her own quite gory severed head. This was not tucked away but was right by the entrance of the Vienna Michaels and unavoidable as you came into the store. A couple of other parents have mentioned it to me too as something they thought was far too gross and realistic to encounter when just walking into a craft store with your little kids along. Tell your friend to be aware of this kind of thing if her son scares easily. I've even seen "severed heads" with lots of realistic bones, blood, etc. sitting on eye-level shelves in drugstores. No thought for younger kids at all. Sorry to get off the topic, I just want folks to know this crud is out there where you might encounter it without expecting it as you might at party store.