The 36-year-old singer - who was the first winner of the music mogul's talent show 'American Idol' in 2002 - hosted the Billboard Music Awards at the MGM Grand Garden Arena in Las Vegas on Sunday (20.05.18) and enjoyed a reunion with her former mentor after she performed 'Whole Lotta Woman'.
Sharing a hug with Kelly, Simon - who was there to introduce Camila Cabello's performance - praised Kelly's set, saying: ''Hi everyone.
''That was bloody fantastic. What can I say? I'm very proud of you.''
The 58-year-old music mogul quit 'American Idol' in 2010 to produce the US edition of 'The X Factor', which discovered Fifth Harmony, 'Havana' hitmaker Camila's former group.
Meanwhile, Kelly - who is now a coach on rival talent show 'The Voice' - made a poignant speech calling for action against gun crime in the US, following the recent school shooting in her home state of Texas.
Ten people lost their lives and 13 were left injured after a gunman fired shots on Friday (18.05.18) at Santa Fe High School in the US state.
Kelly - who has two-year-old son Remington and three-year-old daughter River with husband Brandon Blackstock - said that holding a moments silence for the victims would be pointless and she called for people to start making a change instead.
She said: ''Tonight, they wanted me to say that obviously we want to pray for all the victims and pray for their families.
''But they also wanted me to do a moment of silence.
And I'm so sick of moments of silence. It's not working. Obviously.''
Trying to hold back her tears, she continued: ''Why don't we not do a moment of silence?
''Why don't we do a moment of action, a moment of change?
''Why don't we change what's happening? Mommas and daddies should be able to send their kids to school, to church, to movie theatres, to clubs, you should be able to live your life without that kind of fear.
''We need to do better.
''We're failing our children, we're failing our communities, we're failing their families.
''Let's have a moment of action, let's have a moment of change.''
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