Because of that stimulants (actually amphetamines, don't know about other classes) have more of a calming effect as opposed to a real stimulating effect
I think you might be misinterpreting the meaning of a 'stimulant'. A stimulant doesn't necessarily mean you become psychologically hyperactive and bouncy (but of course this can happen), it simply means the drug tends to speed up peripheral and central processing as opposed to slow it down. Some people have similar confusions regarding alcohol because it's regarded as a 'depressant'. Of course alcohol doesn't make you feel depressed, it just depresses and slows down peripheral and central processing, hence why your reaction time is slower, your movement becomes less coordinated, you slur your speech etc.
What she told me is that people with ADHD usually have some form of defect in the dopamine system
I've actually wrote papers and delivered presentations on ADHD so I know what is hypothesized to occur at the neurobiological level. The dysfunction is thought to reflect the DA/NE attentional networks of frontal, parietal and striatal areas of the brain (if you're more interested in learning the networks then I can discuss it further via PM or something). Rat studies show that the neurochemical balance of DA/NE is out of sync and human studies show decreased connectivity and activation of the brain areas. All of these areas are explicitly involved in things such as maintaining attention, decision-making, reward and social interaction. Anyway I'm going off on a tangent.
People that do not have ADHD and take ADHD medication experience real stimulation. Of course this brings with it an increase in focus also. But the effects are different.
Calming, which increases focus for people with racing thoughts (one of the marks of ADHD)<->stimulating, which increases focus for 'normal' people, but would decrease it if you already have racing thoughts. This is the case for me too. If I take ADHD medication, I relax, my thoughts relax and this in turn increases my focus by a tenfold. If I give it to a 'normal' mate of mine, he gets all hyperactive and jittery, but because his thoughts speed up he can actually think faster and more focused too.
Kind of addressed this before, you're misinterpreting the meaning of stimulant. The reason the stimulants work on people with ADHD is because they hit the networks I just discussed, where people with true ADHD have deficiencies. The balance between DA/NE now becomes governed by the drug hence why it's therapeutic. People with ADHD are not immune to overstimulation just because they have ADHD. They are just as susceptible as anyone to peripheral overload, the effects are just likely to be somewhat different centrally. I've never been diagnosed, and I don't believe I have ADHD, but I do find the stimulants to produce a calming yet stimulating effect. I've also found them to make me somewhat hyperactive at times. Methylphenidate is particularly more peripheral and jittery than adderall in my experiences. Point is, stimulants can make healthy people calm and focused too. I'm not entirely sure, but I'd imagine that some people with ADHD can also become hyperactive on them. I'd say it's rather dose/setting/individual-dependent of each drug.
Anyway, relating back to MDMA, the action of MDMA is very different from ADHD stimulants. The selectivity for serotonin really doesn't directly cross with ADHD pathology, apart from the partial release of DA/NE. This is even more complex considering that serotonin actually somewhat controls DA/NE systems and so part of the release into these systems is downstream from colossal 5-HT efflux. So it's hard to know what direct action MDMA is actually taking on DA/NE and so what direct effect having ADHD will play on the MDMA experience. Pure MDMA itself is can be rather relaxing anyway, so even if someone with ADHD says it 'calms them down' this effect happens in perfectly healthy people too.
Basically, I'd place my bets on MDMA not being as 'stimulating' to people with ADHD because of strong developed tolerance to stimulants from years of daily usage. I would hypothesize that if someone is ADHD, but stimulant-naiive, then they would react very similarly to MDMA as a perfectly healthy person.