IIRC, the assault occurred because there was a hole in the wall that allowed the other prisoner access to Damien's cell. When he told the guards, they did nothing to help him. He discusses it in Almost Home, and it's been a while since I read it.
ETA: Sorry, I just looked back at Almost Home. It doesn't discuss the rapes only the abuse by prison guards. I guess it was in one of his many interviews that he discussed the rapes.
Thanks for informing me of the details, as I said in my post I wasn't sure what had happened, but I did know that DE said he had been repeatedly raped, so I'm glad you gave more detail.
My last post was mainly to let the poster who said that they thought rape was unlikely to happen (sorry I forget who said that) on DR's because of prisoners being isolated, that there had been other prisoners on other DR's who had been raped, because they were - or used to be in the past, times when DR prisoners did mix.
When I remember the case, I will edit this post to give the example.