Modifying the trigger by smoothing its surface, or by smoothing the gun's action, seems fine to me. I've never run across a case of opposing counsel claiming recklessness on the user's part because the trigger was too SMOOTH, but can't count how many times it was argued that the trigger was too LIGHT.
Remind your friend that even if he keeps his finger off the trigger, that won't prevent a false allegation of unintended "hair trigger" discharge to support a bogus theory of Manslaughter or Wrongful Death. Unless the shooting was videotaped from the right angle, there will only be his word that he didn't fire by accident...and it will be tough to prove that he's a perfect human being incapable of making a mistake.
Stay within manufacturer's specs for duty guns, and you should be very defensible. In the case of the Glock, the manufacturer has made it clear that this is in the 5.5-pound pull weight range, either the standard trigger in the commercial guns, or the 3.5/4.5 lb. connector mated with the NY-1 module.
For an idea of how courts are likely to look at a 3.5/4.5 pound trigger pull, your friend might want to look at Santibanes v. City of Tomball, TX, 654 F. Supp 2d 593, 605-06 (S.D. Tex 2009), involving an unintended discharge of a Glock 21 with 3.5 pound connector, reported some months ago by an attorney here on Glock Talk.