Bono is Troy's best friend and sidekick. It's been that way ever since they first met as young men in prison. These two pals work together as garbage men and hang out, sipping on gin every Friday night. The Friday-night drinking sessions we see make it pretty clear that Bono is second in command of the pair. Mostly he sits around and nods as Troy talks and talks.
Bono admits that he's always admired Troy, and that he's learned a lot by following him. He says, "I done learned a whole heap of things about life watching you. I done learned how to tell where the shit lies. How to tell it from the alfalfa" (2.1.38). Bono also tells his friend, "You done learned me a lot of things. You showed me how to not make the same mistakes...to take life as it comes along and keep putting one foot in front of the other" (2.1.38). With lines like that, it seems like Bono is definitely the sidekick in this situation.
What's interesting, though, is that by the end of the play Bono and Troy don't really hang out anymore. It's never said outright, but it seems pretty clear that this is because of Troy's affair with Alberta. From the very first scene, Bono is trying to steer Troy away from this sexy lady. Troy, however, doesn't listen and has the affair anyway. The last time we see the two friends together, it's clear they don't chill anymore. Troy's promotion to driver has separated them at work, and Troy's betrayal of Rose has separated them on a personal level. It seems the affair damaged Bono's admiration of Troy. In the end, Troy hasn't just lost his family; he's lost his best friend.