Set some alarms.
Being late for an ADHD person is not an act of maliciousness, and being highly aware that your lateness causes others annoyance, inconvenience, and harm mostly causes anxiety and self esteem issues, rather than fixing the lateness. Time blindness is common in ADHD. It’s really hard to keep track of time without external help. Why they are late will inform some of the things they can do to help this. Are they forgetting that they need to be somewhere at a certain time? Set a series of alarms at intervals before they need to leave (an hour before, a half hour before, 15 minutes, 5 minutes, and time to go). Are they aware they need to leave at a certain time, but get distracted by other things? Leaving less time to get ready actually helps me not get distracted. Are they forgetting things that they have to go back for? Laying things out beforehand and having dedicated locations for necessary things is helpful.
You can train yourself to think about schedules in terms of when you need to leave or when you need to start something, but the default tends to be thinking about it in terms of when you need to be there or when it needs to be done, and our brains just don’t deal as well with that. For example: I need to be there at 8. Well, the fastest I’ve ever gotten there is 45 minutes, and so that becomes the default time in my brain, and I figure I’d better be going by 7:15 – except I’ve forgotten that brushing my teeth takes 5 minutes – it’s a 2 minute timer, how could it possibly take longer than 2 minutes? And I’ve forgotten that it actually takes time to gather my things, even if I’ve set them out, and walk out to the car – just because I’m ready to go at 7:15 doesn’t mean that’s the time I’m leaving the driveway. So I’m leaving at like 7:25, but that’s ok, right, that’s not that late – except that the last time I made it in 45 minutes was at a different time of day and there was no traffic and no weather, and somehow it takes closer to an hour to get there, so I got there 20 minutes late. Again. And I can’t seem to make my brain believe that walking to the car takes time or that the trip actually should take an hour. And it doesn’t make any difference that people are waiting on me or that I’m going to get in trouble for being late, because if those things made a difference, I’d have solved this issue 3 decades and a couple of anxiety attacks ago. I do better when I tell myself I have to leave the house at 7 rather than I have to be there at 8, because that gives my brain less room to be overly optimistic. I tell myself I have to be in the shower by 6, rather than I need to be finished showering by 6:30 – because I have no concept of how long I’m taking once I’m in there. I try to leave myself with only the time that I need to get there, because if I have 10 minutes that I don’t absolutely need, I’m going to spend 20 minutes doing dishes or changing the laundry loads quick before I leave. I set 3 alarms to get up and two to leave the house. And I’ve never kept a job that was strict about time or friends who weren’t understanding and tolerant, because I’m not actually a masochist.