Well, there's a number of things you can do. I've probably forgotten most of it because I haven't been scripting for well over a year.
1) Max the skill of the sniper.
2) Add binoculars to the sniper.
3) Reveal the targets. Check out the command reveal in the ComRef.
4) Use spotters in the same group as the sniper. If you're really nasty you can have the spotter(s) someplace entirely different, like closer to the target than the sniper. Just make sure he isn't detected.
5) Put the sniper on a guard waypoint. I have no fecking idea if that helps, but OFP AI is at it's best when on a guard WP. ;D