"After careful consideration of all of the relevant material, I have concluded that Mr McKinnon's extradition would give rise to such a high risk of him ending his life that a decision to extradite would be incompatible with Mr McKinnon's human rights," she told MPs.
A so-called forum bar will also be introduced to extradition proceedings to enable British courts to decide whether a person should stand trial in the UK or abroad, Mrs May added.
Ms Sharp said: "I know it's been a life-saving decision because Gary doesn't travel abroad, he doesn't go on holiday, he very rarely leaves north London, and to be taken from everything you know, your family, everything, thousands of miles away is so terrifying to him.
"I can understand that he felt he would rather be dead."
The last 10 years had been "horrendous" because her son would "just sit in the dark all the time", she added.