Coffee has NOTHING to do with Ritalin or Adderall. It's going to make his problem worse- Ritalin works by increasing dopamine levels in the brain (to make it simple), and caffeine does the exact opposite.
If you think that caffeine is an effective dopamine antagonist, then I'd say you have a very poor grasp of the neurological basis of addiction and the reward pathway in general. Amphetamines & caffeine are related compounds functionally, if not that similar molecularly. Taken together, they even potentiate the other's effect.
I can reference some scientific papers that delineate this if you'd rather. I don't post completely uninformed opinions on the Internet. I study neuroscience.
/science rant over/