Your friend is somewhat wrong. Språkrådet recommends writing it apart and says that although the form hejdå is common in practice, it's still only considered an informal version.
http://www.spraknamnden.se/fragor/arkiv_sprakrad_05.htm#hejda Both versions are accepted answers here though :)
"Hej" and "då" should NOT be apart, at least in Skania it's like that. Since my childhood I've always said "Hejdå", but I'm still confused on why Duolingo said "Hej då". I didn't get incorrect, instead Correct.
I think it's because in Sweden people speak a little different if you live in the Northern area or Southern area.
I would say "Hejdå"
Earlier you told me the correct answer, right? Where do you get theses correct answers, do you get it from yourself, duolingo or a website. Because, it could be helpful for both me and others. I'm not being rude, just wondering where you got these answer so I can remember what's right and wrong:-) I hope you understand!
That depends on what you mean. Much of what I say is simply because I know the rules of grammar and spelling well. I'm also a contributor, so if I say that an answer is accepted, it's because I've checked in the admin interface. If need be, I can look definitions and etymologies up in dictionaries.