Given this plot summary from The Night Buffalo, what caused Manuel to discover these occurrences caused by his late friend?

The Night Buffalo (orig. Spanish El Búfalo de la Noche) is a novel by Guillermo Arriaga.

Plot summary
After Gregorio commits suicide, his friend Manuel finds himself unraveling his late friend’s world, and what led him to suicide. Gregorio’s tortuous relationship with his girlfriend is now inherited by Manuel; he becomes involved with his late friend’s girlfriend. Gregorio has missed appointments, left strange messages, and has been harassed by a vengeful policeman.

Gregorio committed suicide, this caused Manuel to go down a path of discoveries uncovering what led him to suicide.