A Ramos Gin Fizz uses both egg white and heavy cream in order to form an extraordinarily stable and dense head on the drink.
When egg white is whipped, it forms a foam through protein networking. When cream is whipped, it forms a foam because fats capture air in bubbles. This question tells us that whipping them together doesn't work because the mechanisms interfere.
Furthermore, we're working in an acidic, alcoholic environment. This ATK article explains that cream thickens instead of curdling because the fats (again) interfere with the protein networking action, this time of the caseins.
I've gotten good results on the foam even when I omit egg white, meaning that it's probably not an integral part of whatever chemistry is causing the foam. I've also noticed that I get better results if I mix the club soda and cocktail streams as I pour them, making me think there's something of an emulsion happening during the pour, and the gas in the foam starts as mostly CO2.
The Cocktail Codex by Death and Co. contains a recipe for the RGF made in an iSi whipper, so whatever's happening, it works just as well with CO2 as it does with N2O.
What's causing the foam to be so stable?