I'll try to KISS on this one. In years past it was an automatic disqualifyer and very few waiver were given. Now, probably because it is becoming such a common diagnosis (or misdiagnosis) they're a little more lenient.
ADHD in and of itself today generally won't disqualify you from military service. But you have to remember, each branch of service has their on policy on this. You'd have a better chance of getting into the Army with a history of ADHD than the Marine Corps, for example. That isn't to say the Marine Corps won't take you (see Rhode's post) but it's harder to get in. Plus, a lot of people lie about it on their med forms. Do not lie, ever. It'll come back to bite you in the fourth point.
There are a few different types of ADHA (inattentive, compulsive, combined) and that would have a factor as well as the severity and current status. You have to no longer display any active sympotoms of ADHD upon enlistment, and if there's any medication being given, you have to be off of it for at least 12 months prior to your physical. The reason being is the military policy of disqualification for the use of daily medication applies to any chronic disorder or condition that requires daily medication (such as medication for thyroid disorders, asthma, and diabetes, as well as stimulant medication for ADHD.)
The MEPS doctors (most of them) are pretty good about asking you questions to determine if you're fit. I would recommend if you know someone that's actually been diagnosed to get all of his medical paper work together, as well as any statements he can get from his GP and Psych stating he's good to go and can function without his medication (if he was on it recently). Also any school records that show a good academic and behavioral record. Give them to the recruiter and let him read over them, if he's not sure, tell him to do a med review before he goes to MEPS. This will let the doctors look over the paper work and clear him for a physical.
The more prepared, the better the outcome and any time you're dealing with MEPS you can never be too prepared.