Translation:A best friend is a person's best friend.
Is there any difference between the meanings of "bestevenn" and "beste venn"?
Yes, the first is a noun, the second is and superlative adjective and a noun. English is weird in that it may use a space in a compound word, such as here, so it's not really possible to tell the difference.
My understanding was that when an adjective and a noun combine to create a new word, it is considered an open compound word. E.g., living room is a completely different concept than simply "living" + "room". That doesn't seem to apply to "best friend".
The idea of a "living room" that wasn't an open compound word is slightly horrifying.
It's an open compound word if there's a space between them. (Example: ice cream)
It's a compound word because "best friend" has a specific meaning, she/he's (one of) your closest friend(s). It doesn't mean she/he is the 'best' in the sense that they're the best in a competition. You wouldn't say "she's my best friend because he's best in everything." :)
Admittedly whether or not a word is compounded or not, is debatable in English, because it's not as easy to distinguish between two words and a compound word. The line is sometimes blurry.
Compounding words is also much more common in Norwegian. 'rødmaling' is a compound word, but would 'red paint' be considered one? Probably not.
I had never heard of open compound words. I tutor English as a second language, and this term will be useful when I try to explain this to my students ( ie White House versus white house.)
Old English used many compounds because separating them would require every word in the chain of meaning to be inflected. After English lost its inflections (especially on adjectives), there was less need for compounds. That has to be very weird for non-native speakers.
This is a really silly sentence, but to explain in a less grammatic way: I consider that you can have several "bestfriends", e.g. your bestfriend from school, your bestfriend from uni, and a sibling you're very close with. It's like a title you can bestow upon several people. Meanwhile, your best friend would be your very best friend. The one you rank over all other friends, like in English.