The TV producer and his former partner arrived in his Rolls-Royce Phantom at the Beverly Hills Hilton in Los Angeles, reports The Sun.
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".