Alex tells Bob who to look out for and who not to talk to, and protects Bob while in with him.
Whenever a new prisoner or guard arrives, the Warden warns him that rapists will not be tolerated and that anyone caught committing rape will be summarily executed in the courtyard. The Warden also makes them watch old videos of these executions to show that he means business.
Discussed: "Poor Bob! How will he survive the next 15 years?!"
Conversed: "He'd better hope he gets parole..."
Implied: Shortly after his prison sentence begins, Bob begins walking with a limp and complaining of pain in his anus. He also has some... suspiciously-located bruises.
That's how Bob started raping people (men, women or children) in the first place. He wanted to establish dominance to cope with the trauma.
Reconstructed: Eventually, Bob finds someone that takes him seriously, and gets the help he needs. He eventually goes on to lead a normal (or normal-ish, depending on the length of his prison sentence) life.