There's a lot of ignorant people on this thread.
Time blindness is part of having ADHD. You have no internal clock. It's one factor which means people with ADHD can be late often and also they means they find it very difficult to manage their time.
As an employer you have a responsibility to make reasonable adjustments. By definition these should be reasonable and I'm not sure what's being requested is as it would have a substantial impact on other people's time/day. You can't have people waiting around constantly for someone.
What helps with time blindness is clocks everywhere. People with ADHD should have prominent clocks everywhere and one on their wrist. In addition, you need alarms and reminders. Again they need to get clear and prominent. If you use outlook, the reminders on their are fucking pants for someone with ADHD. Too cluttered and so it's v hard to pick anything out so you tend to just click it away without realising.
I would say a reasonable adjustment could be prominent desk clock, a decent reminder system for meetings, and some additional allowance if systems fail and the person is late. Systems and strategies are helpful but they don't get rid of ADHD so if you rely on them and they fail you will fail. Some understanding when that happens would be very appreciated (certainly by me anyway!!).
Please bear in mind that ADHDers have to set A LOT of alarms and reminders. Too many gets overwhelming, stressful and you can start ignoring all of them when they are going off constantly. They therefore have to be prioritised. So important things get perhaps a calendar item, a to do, and multiple alarms, if it's less important it gets one alarm and just stored in one place, if it's not that important it just goes on a list(s) that I try to check every few days. I might set alarms in two places. For example on my work computer and on my personal mobile.
Sometimes you have to fuck things up a few times to realise that something needs a higher priority! And sometimes you have to play around with calendar items, to do lists and alarms to work out what works. ADHDers find it difficult to work this stuff out in our heads, we have to do by trial and error.
It would also be so helpful if people understood the effort that all takes and how long it takes. It's time consuming (and difficult) to manage your executive functions outside your head. Any support or technology you are able to provide as an employer which support with that is going to help your employee be less stressed and more productive.
It may be an idea to access an Access to work assessment for your employee. They can do an assessment to suggest reasonable adjustments and help fund them.
To be fair to your employee, they may be asking for this as they don't actually know what's available to support them. I hear this a lot on ADHD forums. People ask for the "wrong" things at work (or don't know what they can ask for at all conversely) because they don't understand what's available and how to access it.