What are some words or phrases for "friend" in Spanish slang?

Answer 1

"Mano," short for hermano, at least in Mexico.

And "tío," which means uncle, is common in Spain.

And I once heard someone in Peru address another as "brother" when asking for a favor, so I guess that works too sometimes! (Just say it like you only speak Spanish; if you pronounce as a native English speaker, they might not recognize it.)

Answer 2

Cuate 

Viejo

Gallo -->used more in the north of mexico with a close friend