He is probably not done with you. He is just dealing with the fallout of what you told him, and right now that emotion is pointed in your direction because you were the one who delivered the news.
That is a common reaction and it is rarely permanent. When someone hears something painful about a person they care about, the initial response is often to push away whoever broke the news rather than sit with what the news actually means. It is easier in the moment to be angry at you than to process what his girlfriend may have done.
You were looking out for him. That matters, and at some point when the dust settles he will likely recognize it. People who are genuinely holding someone down through an incarceration are not easy to find, and inmates who have that kind of support generally know the value of it even when emotions are running hot.
Give him some time and space. Do not chase the conversation or keep pressing the point about what you saw. You said what you felt he needed to know and that was the right call. What he does with it is his business now.
Reach out once after a reasonable cooling off period, keep it brief and genuine, and let him know you are still there. More likely than not he comes back around once he has had time to think clearly.