Hmmm. I'm guessing the problem is with his front brake, moreso than the disc. In the wet, the rear discs typically have more 'bite' than the rear wheel has traction to use it and you simply skid the rear wheel.

Regardless, I'd suggest having him check the "freshness date" on his brake blocks (if they're old, consider new ones with more bite), do the brake track cleaning think, and also check rear disc for proper adjustment, pad wear, and proper rotor centering per Avid's instructions.