A80 Controller providing OJT described a loss of separation event when traffic departing on an RNAV SID experienced equipment difficulties straying from the published route and conflicted with parallel runway departure traffic.

Narrative

I was conducting OJT on departure RADAR South. Air Carrier X departed Runway 27L; Runway 27R; the normal departure runway had a disabled aircraft on it; on the NOV RNAV SID. It was apparent Air Carrier X was south of the RNAV tracked. A few miles behind him Air Carrier Y departed on the DAW RNAV SID on track. Air Carrier X checked on stating he was having issues with his compass and would be deviating from the RNAV departure. Air Carrier X had already compromised separation between himself and Air Carrier Y. Unfortunately the Developmental's reply was 'roger'. I immediately told him to turn Air Carrier X and he assigned a 270 heading. Air Carrier Y was still on the Tower frequency. I promptly called Local Control 3 and told him Air Carrier X had compass issues; was assigned a 270 and to assign Air Carrier Y a 255 heading to ensure separation. A80 has a waiver for reduced departure separation. Thankfully; DR-N had no departures on the westbound RNAV heading because assigning. Air Carrier X a 270 vector would have compromised separation between those 2 tracks. ATL Tower needs to ensure aircraft are on the correct track; hold onto the aircraft until this occurs. In this case if Tower had both aircraft on frequency visual separation could have been applied. Air Carrier X should have advised the Tower of his situation. NOT check on departure already deviating into the path of Air Carrier Y.