What would convince him to switch? He should just tell the doctor he heard or has a friend that said adderall was more effective for them, or that he was researching ADD drugs and wanted to inquire about Adderall/possibly try it. I have ADD and have used this method of trying out different drugs effectively.
On a side note to those who responded: you can't just say, "If he has ADD ritalin should work", because first of all he never said it didn't work for his friend. People need to find what works best for them. Adderall and ritalin are completely different stimulants, and their level of effectiveness is different for everyone. Some find ritalin works better, some Adderall, some otherwise. Hell Wellbutrin works for some. I personally prefer dexadrine, and yes I obtain it legitimately. They don't usually prescribe that as frequently, but Adderall should be no biggie, just have him tell the doctor something to the effect that he thinks it may be more helpful and beneficial.