Cowell, 50, has been acting "teenagery and happy and light," British actress Amanda Holden tells PEOPLE. "They are like Tweedledee and Tweedledum. You can't separate them really. "
Holden, who is a Britain's Got Talent judge and CBS Early Show special correspondent, might surprise many Cowell-watchers by saying that her friend is "shy" and not prone to outward displays of his private thoughts. This may explain why he and Afghanistan-born Hussainy, 36, have been so coy about their marriage plans.
"Apart from his mum [speaking about him], he's never really let on much about his private life," she says. "He has to maintain some kind of privacy in such a public arena. He wants to keep something back for himself. He's an incredibly shy person."
Holden can't confirm that Cowell is engaged but what is clear is the ring that he has bought his love – a huge diamond sparkler that has been turning heads. "It's the Rock of Gibraltar – it's massive, it's huge," Holden laughs. "You could get a whole community living on there."
Holden adds: "I don't see the point of Simon's success unless you have someone in your life."
Why may Hussainy be the perfect someone for Simon? Besides being independent, gorgeous, sexy and funny, she says what she thinks and that works for Cowell. "People who he trusts the most are the people who talk to him properly," Holden says. "He likes proper opinion and she's very strong and definitely vocal and honest. Most people in his life are ... He's a straight talker on the telly and real life."
"Simon's fallen hook line and sinker," Holden concludes.