It happens a lot with ADHD, or any kind of difference for that matter, that such a person will have fewer friends. In my own experience, that has meant fewer, but higher quality friends.
My speculation based on limited information:
I know it was annoying, but he really was trying to make the most of it, which to him meant staying up all night and doing all the things he normally gets a “no” for — one final epic adventure with his pal. His friend probably would have liked it too, except he didn’t like the resulting conflict it created — kids without ADHD are less prone to feed off of conflict. I don’t know your son, or how well he reacts to the occasional allowance of rule breaking, but in this case it might have worked out better if you cut him some slack and let him stay up all night with a friend he needs to say good bye to (even if just by looking the other way rather than explicitly saying he’s allowed). We ADHDers often need things to be extreme when we have emotions to deal with (because our emotions are extreme in these moments). When all else fails, conflict is always available to make it extreme. So, whatever you can provide other than conflict will really help. Giving him the impression that he got away with something could have provided a less stressful outlet for everyone.
All that said, I know there are kids that will take a mile when you give an inch. So this may not be what would work for your son.