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 B��falo 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.