Richardson, who wowed the judges on Saturday with his 'I'm A Believer'/'Hey Ya!' mash-up, confessed that he ended up screaming when the incident happened in rehearsals.
Speaking to X Magazine, he said: "Rihanna showed me her bum last week. She was dancing and was like, 'Can you see my booty?'
Meanwhile, the 19-year-old wildcard act also confessed that he nearly quit the show following the death of his grandmother.
"Rebecca and Mary stopped me going. I just sat down with them and they're both so warm and loving, I felt happy again," he said. "I didn't even have to tell them how I was feeling. I decided I had to stop wallowing. I had to pull myself together."