My friend told me that Ramsay flayed Theon; this question is in two parts:
- Is this correct? and
- What else has Ramsay possibly done?
I have not read the books, but don't mind spoilers from them.
Ho boy. Where to begin...
He knew he ought to hide the rat, but he was so hungry. It had been two days since he had eaten, or maybe three. Down here in the dark it was hard to tell. Though his arms and legs were thin as reeds, his belly was swollen and hollow, and ached so much that he found he could not sleep. (DwD, Reek 1)
Knocking his teeth out
“He smiles less often now,” Lord Ramsay confessed. “I may have broken some of his pretty white teeth.” (DwD, Reek 1)
Slicing off fingers and toes
Thus far he had lost two fingers off his left hand and the pinky off his right, but only the little toe off his right foot against three from his left. (DwD, Reek 1)
But its how he cuts them off that's the kicker.
Lord Ramsay would never simply cut off a man’s finger. He preferred to flay it and let the exposed flesh dry and crack and fester. Reek had been whipped and racked and cut, but there was no pain half so excruciating as the pain that followed flaying. It was the sort of pain that drove men mad, and it could not be endured for long. Soon or late the victim would scream, “Please, no more, no more, stop it hurting, cut it off,” and Lord Ramsay would oblige. (DwD, Reek 1)
Assorted whipping and flaying
See the second quote above.
It's implied fairly strongly in the books that Ramsay has cut off Theon's, uh, Little Ironborn. In the show, they make it explicit.
Ramsay rose, the firelight shining on his face. “Reek, get over here. Get her ready for me.”
For a moment he did not understand. “I ... do you mean ... m’lord, I have no ... I ...”
“With your mouth,” Lord Ramsay said. (DwD, The Prince in Winterfell)
Most notably, Ramsay intentionally allowed Theon and a woman named Kyra to escape, only so he could hunt them down and drag them back to the castle. But obviously, the nonstop torture didn't help. By the end, he makes Theon forget his own name, crafts an entirely new identity, and breaks the smiling prince into a writhing creature terrified of Ramsay. There are literally too many instances of this psychological torture to quote.