Speaking from 66 months of direct experience, a one-year sentence is on the lighter end of what the system hands out, and the environment that comes with it reflects that. Short-term inmates are typically placed in minimum or low-security facilities where the general population is made up of people who are also close to the door and have no interest in making trouble. Everyone is counting down and most people just want to get through it cleanly.
Prison rape is a real thing, but it is far more common in higher-security settings with longer-term populations. In a minimum-security facility with short-timers, the risk drops significantly. That does not mean zero, but it is not the daily reality that the worst-case portrayals suggest.
Will it break him down? One year is not long enough for that, assuming he finds a routine and stays out of other people's business. The first few weeks are the hardest adjustment. After that, the days start to move and the routine becomes the thing that carries him through. He will have more time to read than he has ever had in his life, and if he uses it, he can come out with real knowledge and perspective he did not have going in.
The experience can be eye-opening or it can be wasted time. That is entirely up to him. If he is the kind of person who keeps busy and stays focused, this will be hard but manageable, and with any luck, he will take enough away from it that he does not have to do it again.
Stay connected through letters, calls, and visits. That connection matters more than anything else during a stretch like this.