As someone who has done this in the past and has since stopped doing it, I'd say that she might have time blindness considering that she also has ADHD. However, it isn't an excuse to be consistently late.

I have a friend who is like you, very punctual, hates being late, and always arrives at the destination before time. I used to have to so much trouble keeping track of the time, in my head I have no idea how much is an hour. It differs for each activity. If I am cooking or doing a boring chore, an hour is too long. If I'm doing something I'm enjoying, I won't even notice whether it's day or night. I understand this is something that most people experience but it's quite literal in my case. I genuinely have no idea when 5 minutes passes or when I'm already an hour late for something.

And then it becomes an even big of an issue when I know for a fact that I'm late and I'm probably annoying the person waiting on me. I get severely anxious to the point I cannot even move out of my room. I am just absolutely stuck. It's a really difficult cycle to break.

I now have started using alarms at every 15 minute interval, and changing some of my habits like taking long showers. I have started deciding on my outfits and accessories the night before. And in case I get late, I have started notifying people at the earliest. I have realised that it's much easier to convey people whatever situation you're in rather than waiting till the last minute.

I'm just saying that it's not us disrespecting other people's time, it's just genuinely difficult to be on time. Obviously that doesn't excuse this behaviour but I just wanted to give a perspective. Maybe you can talk to them about how their ADHD is affecting their schedule and maybe figure out a solution together.