It depends. In most cases, 'Heaven' and 'Hell' are capitalized when referring to the specific religious or mythological concepts. But if used in a more general or metaphorical sense, they might not be.
To be honest, it's hard to say. Sometimes stories like this have elements of truth mixed with imagination. You'd have to do some research or ask the creator to know for sure.
It could be a fan - made story that combines the Naruto universe with the concepts of harem (where one character is surrounded by multiple love interests), heaven (maybe a positive or ideal setting), and yandere (a character type that is overly possessive and often violent in love). However, without reading it, it's hard to say precisely.
The characters of the best friends are important. Their personalities will determine how they react to being in 'hell' or 'heaven'. If one is very optimistic, they might see a way out of 'hell' more easily. Also, conflict is a key element. There could be conflict between the friends because of their different situations, or conflict within each friend as they struggle in their respective places. And finally, there's the resolution. How do they come together in the end? Do they both end up in a middle ground or does one pull the other out of their situation?
It might be a fantasy story. Maybe the two best friends are transported to two different realms, one to hell and the other to heaven. In hell, there could be all sorts of terrifying creatures and challenges, and in heaven, it's all beauty and peace. They then have to find a way back to each other and the real world, and their journey shows the strength of their friendship.