I am a Step Mom to a 7 yr old ADHD child.
Hi, and welcome!
So he doesn't act up when he is with you AND the meds are wearing off? It could very well be a med issue, or maybe an overload if she is giving him more meds, since they are generally ONCE a day meds. What med is he taking?
Kids generally "unload" on those they either feel most comfy with, or those they know they can get away with it with.
Does he see a Psychiatrist? If so ask about behavior therapy, and make her participate since she is the one having the issues. I would also set FIRM limits, and make them BOTH stick to them, no questions, no deviations. Have her read 123 Magic, maybe it will help also.
A child may HAVE ADHD, but it is not what they ARE. Never tell a child they ARE ADHD.
Except, this world of ADHD is NOT simple, or cut&dried. From what you're saying, yes, the discipline sounds lacking. Equally, my kid, who's been raised with limits, etc., etc., can have nasty reactions when the meds are wearing off. It's more a matter of being understanding, of making sure he knows how to calm down at that point (and believe me, it's really fairly obvious--I remember him being upset because he knew he shouldn't be so upset!).
I'd also *strongly* recommend that positive discipline techniques--sticker charts, rewards (small!) for good behavior, etc., along with plenty of calm teaching work a lot better than lots of negative consequences. Especially with ADHDers, the research says...basically you can pretty quickly get to the point where the kid doesn't *care* that you're taking X away again.
I'd also wonder, given the circs, if he's actually acting out to get attention?
Anyhow, you can hit any ADHD book & get support for your viewpoint, that's for sure.