Look how quickly the Marauders turned on each other after they graduated Hogwarts.

Their friendship was never that strong to begin with.

Some groups like that bond over whacking the Inferior people/person. Once Snape wasn't available for soccer ball duty, that could be one reason they fell apart.

Sirius can perfectly fine using Lupin as a weapon to kill Snape in book 5, not care about the consequence.

The Marauders didn't merely drift apart. They turned on each other - and way before Oct 31, 1981. Potter and Black suspected Lupin was a traitor (because they thought Lupin betrayed them, and joined with Greyback). Lupin suspected Black was a traitor. Pettigrew was the actual traitor. Pettigrew betrayed Potter and his family to Voldemort, then he framed Black for his crimes.

When Lupin heard that Black betrayed the Potters and murdered Pettigrew, he immediately believed it. That shows us what a low opinion he had of Black.

Lupin lives in a world where it's extremely easy to steal someone's identity (Polyjuice Potion, Transformation), yet he didn't think for a moment that there could have been some foul play involved. He never doubted Black was guilty. He thought betrayal and mass murder were perfectly in Black's character. He didn't care that Black didn't get a proper trial, didn't try to get him legal help, didn't bother to visit him in Azkaban. Lupin is a fair-weather friend. Not just to Black - to Harry, too. And to his own wife.