OP, I think you need to apply some logic.
Looking at your drive, you’ll know roughly how deep the drainage channels are in relation to your neighbour’s property. If water from your drive drains into them with no problem and you don’t have puddles collecting on your drive regularly, how can this stained water have gotten from your drive onto your neighbour’s?
As she is the one threatening court action, she would have to prove to the court that the damage has been caused by you.
You seem to be accepting what she says as being absolutely true. How do you have a not stained area between your drive and hers?
Water does not go upwards. If it’s water from the hardcore which is the foundation of your drive then that level would have to be at the same/slightly higher height than your neighbour’s drive. Your drainage would also have to be not working.
Now, if it’s been a one off that’s caused the damage (evidenced by the lack of continuing staining) then there’s no chance of it happening again so she can get her drive put back to the condition it was in prior ie faded, wonky, weedy 30 year old tarmac. Nothing more.
NDN: You’ve damaged my drive.
NDN provides evidence that the damage was caused by OP
OP: You’re right. I’m sorry. Here’s a solution to put it back the way it was.
NDN: Fine
THE END
Or…
OP repaints drive
NDN: No, I want a new drive now. It’s not nice.
COURT: No
THE END