Incorrect comma usage, actually. You can't omit the Oxford comma (the comma before an "and") with a list of exactly 3 items. If you do, the middle item in the list can be parsed too many different ways, as happened here.
I beg to differ. I don't see how an oxford comma would fix this confusion. In fact I think without it his statement is easier to understand. We can easily see he means three people. If he used the Oxford comma the second person would look identical to a preposition and would make your parents comment more valid.
