It's not just about English. I am a native to this country, and English is my native language, one which I have spoken as long as I've been speaking, and one that I speak rather well.
And yet, at least once when I told a bar patron that we offered a 15% discount to locals, he honestly thought I said 50%. And was stunned when he didn't get it.
Yeah, right. Nice try, bub.
To your non-American coworker's credit, the words "fifteen" and "fifty" DO sound a lot alike. The customer was just an ass monkey.
"The Customer Is Always Right...But The Bartender Decides Who Is Still A Customer."