Hmmm. They've previously tried the screwdriver technique without success (even getting a friend to help), which is what prompted this thread.
I did wonder whether a stop tap key was the appropriate tool, and my contact has been asking around using the photo for reference, again without success. But stop tap keys seem to come in different styles/shapes/sizes, which is why I'm awaiting a report on the slot dimensions etc.....and I was also assuming, perhaps incorrectly, that stop tap keys are for the actual stop tap rather than the cover.
Unfortunately I am doing this remotely, and my contact is a complete airhead on anything like this. And I've only ever had to deal with easy-opening inspection covers for water meters.
Thanks for the replies.