Have you ruled out a sportsmans hernia. This is common in footballers, responds v well to rest but returns when you start activity again.

If it is just an adductor strain, I would use deep transverse frictions on the painful area. You also need to stretch out the muscle and begin a strength programme. The strength programme should consist of isometric contractions while pain is still present moving on to concentric then eccentric after pain has gone. To maintain his fitness, in this case he should stil be able to run in straight lines.