The CHAdeMO Adapter locks to the car, the CHAdeMO Plug from the station, can be disconnected from the adapter, same as a J1772 can be disconnected from the adapter leaving the adapter locked to the car yet the plug from the station free to be "Reused"
So that Leaf driver could disconnect you and juice up.
That's assuming that the the other driver (Leaf or otherwise) can even REACH the DC FC from that other spot, if any.
(As a Leafer w/CHAdeMO inlet) I've personally run into cases where the Leaf driver left their car, they were well below their L2 speed, the they left no contact info and either the cable definitely wouldn't reach to any nearby location (other areas blocked by other cars or other obstacles) OR, I didn't know about their actual need and didn't know if they'd be pissed if I stopped their charge.
Most of the time, I or another person in line ahead of me was totally blocked. Wouldn't have helped to disconnect the Leaf w/driver MIA.
In another case, I was at
Walgreens | San Jose, CA | Electric Car Charging Station | PlugShare and there was a Leafer already charging. I parked right next to him and asked him to plug me in when he was done as I grabbed a (take out) bite to eat. Unfortunately, he finished and found the cable wouldn't reach. It worked out ok though. By the time I returned, he was on his way out... Probably lost 3 mins of charging, tops... but still.
I wouldn't say that the rule should be stay w/your or within sight of it.... if you're going to leave it on CHAdeMO, PLEASE leave contact info. And, if possible, also leave a sign w/criteria as to when it's ok to unplug.
I have a 2-sided sign that I leave on my dash that says this, depending on the DC FC type:
Please feel free to call me if either my car has ramped/tapered down to power level of 17 A (amps) or below OR you urgently need to DC fast charge.
Please feel free to call me if either my car has ramped/tapered down to power level of 7.0 kW or below OR you urgently need to DC fast charge.
followed by my cell #.