Ok, my turn.
Many good picks here so here are my +1s. First, Zombie Raid is my favorite shooter of all time. Second favorite is the sit down Jurassic Park. Its great since you can usually pick it up at auction for a song and a dance. Then, I think Space Gun is my next favorite. I also recall enjoying the SNK shooters like Mechanized Attack, and Beast Busters I and II.
But considering that your friend isn't exactly an arcade collector, his best choices would be T2, Revolution X, or Jurassic Park I or III. Those are games that have great licensing tie ins and are pretty fun to boot. Although, it may be hard to find a good T2 or Revolution X as the usually got played pretty heavily. T2 is interesting because it added to the Terninator story line since part of the game goes into the future so you can fight Skynet. Revolution X=Aerosmith. Very cool if your a fan of the music.
Of course, if your friend is made of money, the best shooter I have ever played is Aliens: Extinction. That is the coolest, hardest, funnest one I know of. Although, Zombie raid is still my favorite.