It took me ages to get over that hurdle. 
Like if someone asked me to get a drink at the bar and they didn't have that exact drink, I just wouldn't get it, and come back and explain they didn't have it. 
Eventually I started asking people what their back-up choice was, then their second back-up choice...  but that kinda annoyed people. 
Then eventually, after a few years, I kinda understood that if what they wanted wasn't there, that I should just... get the nearest equivalent as best I could. 
But yeah... in her position I would have.... well okay, I'd have asked for it to be written down on a post-it note because I can't memorise anything in audio, and then handed the note to the barista...     BUT assuming I didn't have a post-it note...  and at least vaguely remembered what it was....  upon not seeing it on the menu, I'd probably have just given up and walked out.