The TV producer and his former partner arrived in his Rolls-Royce Phantom at the Beverly Hills Hilton in Los Angeles, reports The Sun.
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It is the first time they have been seen in public together since they split earlier this year.
A source stated that the pair were at the hotel for a "promotional event for the US X Factor", saying: "They've always been close and remain on good terms."
Insiders close to Cowell denied that the couple had got back together.
Cowell and Hussainy were in a relationship for two years before their engagement was called off in February.
The make-up artist recently denied reports that she was planning to write a tell-all book about their relationship.
Cowell had previously been quoted as saying that their engagement was a "big mistake".
