Holding the rock, Jon said "I wish that Sarah would feel really sorry for the wishes that she made to affect me. And I wish that Sarah would want to reverse all of those wishes." This way, Sarah would be changing (or specifying) the wishes herself, which wouldn't go against the last wish she made.
Sarah came back into the bedroom.
"I thought you were getting something to eat," Jon said.
"I just can't eat when I feel this bad about what I did to you. I'm so sorry, Jon. Can you ever forgive me?"
"If you fix what you did to me," Jon said.
She smiled. "Yes, yes. I can do that. Give me the rock."
He handed her the wishing rock, knowing that she would help him, because that's what he wished for.