In reply to Simon Lee:
It just looks like a fairly standard cable pull set up with the addition of a smaller handle than usual(i.e. the rock rings).
Think I'd work it the other way though and NOT try to figure out my one rep limit- it's too easy to damage a finger overdoing it.
I'd start with something easy and over 2 or 3 sessions figure out what my 10 rep limit was and start from there.
I'd agree the exercise isn't going to be specific- though to be honest, if you're going a lot of climbing/ campus boarding/ two finger pulls anyway, I'd have thought a bit working something specific (for example a particular weakness) isn't going to be a disadvantage- I assume it's not going to stress the shoulder to much, or the tricep if he's curling that, so it becomes a test of grip- and I'd have thought there are lots of other grip exercises out there. So all in all, I'd wonder where this exercise would come in.