In an emotional call to the Priory Clinic, where the talent show contestant is being treated for exhaustion, the TV empresario told her: "Whatever you decide, I will support you unconditionally", reports the Daily Mirror.
That means the pressure is off for her to participate in the sell-out tour featuring the other finalists on June 12. A friend of Simon's emphasised he'd told her to focus all her energy on getting better.
"Simon told her she had done brilliantly and that she was a great singer who was going to record a great album when she was ready," said the source. "It is just what Susan wanted to hear."
His words of encouragement have come as a relief to Susan's family as she'd spoken to them about her fears of returning home to West Lothian a failure.
It may be some time before she can think about the future of her career, however. The Chief Medical Officer at Priory stressed that patients are rarely admitted for a short-term stay.
"I cannot talk specifically about Susan Boyle, but any admission to a psychiatric hospital for a matter of days is, in my opinion, a failed admission, because either it was unnecessary in the first place or the job hasn't been done fully," said Professor Chris Thompson.
The doctor also suggested the singer's condition was more serious than the public had previously been given to understand. "I read Susan… was assessed under the Mental Health Act. It implies compulsory admission," he said. "It implies there was a degree of personal risk. Secondarily that implies she did not want to come into hospital voluntarily."