There are 2 ways to lighten the SA trigger pull on a S&W.
One is what I feel is the preffered method, changing the rebound spring to a lighter spring. The downside of this method is that it will result in a somewhat "lazy" trigger return that can result in incomplete trigger resets on guns that don't have completely tuned actions. When the trigger doesn't reset completely, it will totally lock up both hammer and trigger until the user remembers to push the trigger forward. Use of a 12 lbs. rebound spring will result in a SA pull around 1.75 lbs. and requires further tuning of the lockwork and mainspring tension to insure reliable reset but it's not difficult to do this. However, even with perfect tuning, the trigger return will be noticeably "lazy" on return when compared to a standard trigger.
The second method, that I do NOT endorse, is to stone the SA sear surface of the trigger on a slight angle in relation to the sear notch on the hammer. This slight change in angle will act a bit like a ramp and provide a bit of "assistance" for a lighter trigger break. IMO it's not a good approach to take, that ramp effect will put all the force of the cocked lockwork on the "point" of the sear notch in the hammer and I believe it will lead to pushoff issues down the road. When that happens, the only recourse is a new hammer and possibly a new trigger. The upside is that you can get a great reduction in SA pull without the "lazy" trigger return of a lightened rebound spring.
Now, as to how you can determine what is the cause of your light SA trigger, for someone experienced with the S&W revolver it's very easy. The lazy trigger return of a lightened rebound spring is very easy to "feel". If someones installed a 12 lbs. rebound spring, or cut the factory spring to this tension, it's about a 5 minute task to replace the rebound spring for someone with the tools and skills to do this.
If the trigger return is quite firm and not at all lazy, that means the sear on the trigger has been stoned. In this case correcting the SA trigger pull becomes more complex and has the potential to get costly. Worst case it means replacing both hammer and trigger and fitting these parts to the gun. Best case is pretty simple, all that needs to be done is to stone the sear surface on the trigger back to the factory configuration, however this does mean that this hardened surface will have been stoned on twice and may not be fully hardened when finished. In that case, pushoff problems will develop in time and require replacement of the trigger. I also think that ANY reshaping of any sear surface should ONLY be done by a qualified gunsmith.
Since it sounds as if both you and your friend are new to revolvers, I would suggest having a gunsmith experienced with S&W revolvers look at the gun. In most areas a swap to a heavier rebound spring should be under 30 bucks. If the trigger and hammer need replacement, it may be as much as a 300 dollar repair, in which case I would suggest a return to the factory. With shipping included, it will cost about the same and you'll be assured the gun times properly and is completely safe.
