SNA Tower Instructor reported that an aircraft did not offset 15 degrees on takeoff as required per the AFD and came close to a commercial airliner also departing; on a parallel runway.

Narrative

I was training on the LC-2; Local 2; position. At this time traffic was moderate with 3 aircraft in the pattern mixed with inbounds and departures. Aircraft Y called up and was assigned an EL Toro Departure; an eastbound departure with an initial heading of 080. My trainee cleared Aircraft Y for takeoff and then proceeded to do other things that needed his attention; such as obtaining visual separation with aircraft on final for the parallel runway; sequencing aircraft for the pattern; as well as issuing wake turbulence and jet blast cautionary for aircraft landing Runway 20L.When Aircraft Y started takeoff roll; Aircraft X was cleared for takeoff on Runway 20R. On the upwind; Aircraft Y drifted slightly right of course into the flightpath of Aircraft X taking off. Both my trainee and I failed to notice this as our attention was diverted to other duties. When the CIC mentioned that Aircraft Y was drifting right of course; I immediately over-keyed my trainee to attempt to rectify the situation but Aircraft X had veered right to avoid Aircraft Y. Aircraft Y failed to offset 15 degrees to the left upon takeoff; which is required per the AFD; Airport Facility Directory. I understand that I failed to notice the drift that occurred on the upwind and the failure of Aircraft Y to offset left of course 15 degrees upon departure. One of the things that I would recommend is that there be signage; such as at intersection Kilo departure on Runway 20L - this is the standard point of departure for most aircraft for Runway 20L - that reminds aircraft to offset left of course 15 degrees upon departure.