How to prevent curdling in a local vegan coffee creamer- aka- Post Apocalypse hippy coffee creamer?
Was the coffee highly acidic or possibly salty? Coagulating agents that I know of consist of apple cider vinegar (which I personally use in my tofu making), magnesium chloride and calcium sulfate. Indeed, according to Wikipedia the common coagulants are salts, acids and enzymes.

Not being a coffee drinker I can't say this has ever happened to me. My boyfriend drinks coffee and I don't think he has ever experienced this, so it might be specific to each coffee. I have had soymilk coagulate in a cocktail I had once, but never when I make soy white Russians, so it must have had to do with the other ingredients in that drink, which have been lost to my memory.

Off topic, but what restaurant were you at?