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.

"이모부" is an uncle who is specifically a husband of one of your maternal aunts (your mom's sisters). Consequently, he is an uncle who is not a blood relation because an uncle who is your mom's brother is a "삼촌", and an uncle who is your dad's brother is either a "큰아버지" or a "작은아버지".

Yes, well I didn't read your question very carefully. So while the answer above is still valid, I will have to defer to Neky about the colloquial use in a restaurant.