Green Arrow isn't typically portrayed as an asshole. He's often a hero with a strong sense of justice, but like any character, he has his flaws and moments of difficult choices.
Sure, The Flash and Green Arrow are friends. They share common goals and help each other out on many occasions. Their friendship adds depth to the comic universe.
Green Arrow's training in the comics was a complex process. It wasn't just one person or event. His own perseverance, along with learning from various challenges and encounters, shaped him into the skilled hero we know.