Muslim scholars have a real answer here. Say so before your friend does.
Four passages say the heavens and the earth were made in six days. They are Quran 7:54, 10:3, 11:7 and 25:59.
Quran 41:9-12 breaks it down. The earth in two days.
Then, thumma, mountains and provisions in four days. Then, thumma, the seven heavens in two days.
Two plus four plus two is eight.
In 41:9-12 the earth is finished before the heavens. In Quran 79:27-30 the heaven is built first and the earth spread out "after that" — ba'da dhalika.
IslamQA 31865, following Ibn Kathir, gives the standard reply. The four days of 41:10 are inclusive.
The earth took two days, and its mountains and provisions took two more, so four means four in total up to that point. Four plus two is six.
Genuinely arguable, and small. The inclusive reading has to fight the word thumma, which marks sequence in Arabic.
Translators who favour the harmony insert glosses that are not in the Arabic. Be honest if it comes up: Christians read the days of Genesis in more than one way too.
Genesis simply does not have this arithmetic problem.
"Two, then four, then two. How do you get six out of that?
I would like to understand it."



They say the four days of 41:10 include the first two, so the total is six.
The standard answer comes from IslamQA 31865, following Tafsir Ibn Kathir on 41:9-12. The four days of 41:10 are inclusive. The earth was created in two days. Its mountains and provisions took two more. So four days means four in total up to that point. Four plus two is six.
Arguable and small: the word then blocks the reading, but do not press hard.
The inclusive reading has to fight the text's own connective. Each stage is introduced with thumma, then, which in Arabic marks sequence rather than overlap. Nobody reads I built the house in two days, then finished the roof in four days as six days in total. That takes special pleading.
Translators who favour the harmony must insert glosses in brackets, such as four days including the former two. Those are not in the Arabic. Note honestly, if it is raised, that Christians read the days of Genesis in more than one way too. But Genesis's numbers at least add up.