I would be more confident of not damaging my Leaf's electronics if the Leaf's battery +ve was disconnected first. It's a normal 12v battery so can support jump starting. Once the other car has started, leave the donor battery connected for a few minutes to charge it back up, especially if the ICE required a lot of cranking.
Also, if you don't disconnect the battery from the Leaf's wiring, the BMS will start the DC-DC converter when the 12v battery voltage drops. A cranking load on it, may overload the converter as well as draining the LiOn battery more than you'd want. (the BMS is not expecting the 12v battery voltage to drop suddenly to well below 12v so you may have warning messages and fault codes logged)