That's an idiomatic usage. They call middle-aged womem in restaurant as 이모 normally, so they called that 이모's husband as 이모부. But that you said is quite a rare case, because in that case, Koreans call him as 사장님 if he is the boss of the restaurant, or just 저기요 as if it is a calling word.
P.S. Korean language doesn't have 2nd singular polite pronoun to call someone who you are talking to; 당신 and 그대 are not like the common "you" in English, so when they use a word like this, just say "저기요" for off-line, and "님" for on-line.