How to make chocolate mousse that will set?
I've never tried the iSi gourmet whip+.
It sounds very novel and like a bit of a gimmicky way to cook which is never a bad thing, especially if you can make your friends say, "wow!"

But the real trick to making mousse is beating air into the egg whites. So you gotta be confident the gourmet whip works, otherwise don't stick with it. I would just use a regular electric egg beater or gasp whip with your hands (it takes about 10 minutes with a good whisk - I've done it and survived, so will you). 

Looking at the recipe, the 1:1 ratio of cream (double whipping is great for mousse) : chocolate is a good way to start. 

If you insist on using your device, here are tips that might help your eggs whip the air needed for a firm mousse: 


Add a pinch of cream of tartar. It helps the peaks hold. 
When your egg whites reach a soft peak, add some 1 tbsp of fine granular sugar and it will give it a glossy sheen
Never over beat your egg whites - soft peak and a bit over is good
And when you incorporate your egg whites into the chocolate-cream mix, put in 1/3 of the beaten egg whites and start mixing to get most of the chocolate, cream and eggs mixed, making sure to eliminate any white streaks. Once it's even, then put the rest 2/3 of whipped egg whites. It usually goes flat and liquidy from over mixing and this is one way to avoid that problem.
if you're not eating it immediately, pop it in the fridge for at least 3 hours to "set". That will stop it flopping into itself, causing it to become liquidy.