They had been friends till X died 5 years ago. I'd use HAD, because technically their friendship came to an end when X died, although poetically I'd say their friendship lasts for ever.

Hi there, good question. It can be a little bit difficult when people talk about people who have passed away. Often when people tell stories about their past or their old family members it is not obvious that these people have died, so it's quite common for the speaker to add a short comment to make it clear, like: 'She has since passed away.' , 'Of course he is no longer with us.' , 'This was long before he died.' etc. So in your sentence, you would use 'X and Y had been friends since they were little.', but just remember that this can be a bit hard to understand if you don't add more information. The sentence by itself might mean that X and Y had a huge argument and stopped being friends. There are other options for expressing this friendship too, for example: 'X and Y were close friends as long as X lived.' 'Throughout X's life he and Y were good friends.' Hope that helps! :)

Had. X died.