First off I can't believe someone would spray a car outside and next to other vehicles...

Usually the best method for removing overspray is as said above a claybar. Have you tried the aggressive clays? Some of the heavy clays are pretty hardcore and although it will more than likely marr the paint it should strip off if you can actually feel the grittiness on the paint.

Possibly a good soaking in panel degreaser too?

Next step would be to take it to a local detailer let them have a look. Personally I would always want to see the car before I quote and maybe even try a little test area to see what I'm up against. Obviously I'm too far away myself...