Sonar is not ideal for scoring to picture, but it works. One thing to watch out for is that Sonar has no ability to lock clips to a timecode location. If you have (for example) sound effects along with your music, and you want to adjust the music's tempo, your sound effects will move in time since they are associated with bars:beats. If you're only doing the music, this shouldn't be too much of a problem.
Have the filmmakers send you a video file if they can. Quicktime Sorensen 3 works well, or motion jpeg, or mpeg. And get 2 pops at the beginning and end. As long as the two pops always line up, you know you are in sync. A 2 pop is a 1 frame long 'beep' 2 seconds before the program starts, and 2 seconds after the program ends. 1 kHz, -18 dB FS sine wave. (the actual level will vary depending on the studio. Could be -20 -24 or anything really...) 2 pops are lifesavers--if you know nothing else about the file just make sure you get the 2 pops.
Having them burn a timecode window into the video file will help as well. And make sure that you know what frame rate you're going to be dealing with and sample rate, usually 48kHz, and if you have to deal with pullup. Sonar can deal with just about every frame rate you're going to encounter. Hopefully they're not shooting HD at 23.976 fps which Sonar has yet to support.