Translation:It is important to help one's friends.
Theoretically, shouldn't ''It is important to help his/her friends.'' be correct as well?
No. "Sin" can only be used to mean "His/Her" if it refers to the subject of the same sentence.
Can sin really be used out of context like this? If I meant one as an impersonal pronoun, wouldn't I want to use "man", not "sin"? And can't "sina", here, refer to any third person name or pronoun whatsoever (mentioned in the conversation, of course), like his, her, their, and it's, etc?
When it doesn't refer to a subject in the same sentence, "Sin" refers to people in general, just like "Man", except "Sin" is a possessive pronoun.
You could translate this sentence as "It's important to help your friends", and it would mean the same thing. Because even though it says "Your", it's not referring to you specifically but that in general it's a good idea for people to help their friends.
But couldn't it also mean any other 3rd person pronoun, within the context of a conversation?
Like "Hennes vänner behöver hennes hjälp. Det är viktigt att hjläpa sin vänner", which would mean "Her friends need her help. It is important to help her friends", right? I know that sentence seems a little weird, but I couldn't think of one that fit the context better.
"Sin" can only refer back to people within the same sentence.
So if you wanted to refer to her friends in that second sentence you would have to use "Hennes", regardless of the context of the conversation.
Does one follow the its/it's rule? I always use the apostrophe for possessive or "one has/is."