Your friend is extremely wrong. The arrival date counts, as Zol​​​​ says.

Emirates does not have fixed tier review dates. You can earn status in any rolling 13 month period. In fact you don't even have to wait until the end of the month. If your flight lands on 15 Dec and you then have enough miles in the past 13 calendar months you'll be upgraded a tier. (I say calendar months because it does look at whole months, so 15 Dec means it'll look at 1 Dec 2017 - 15 Dec 2018, not 15 Nov 2017 - 15 Dec 2018