The on-off girlfriend of the 26-year-old prince has been invited to the nuptials of Prince William and Kate Middleton at London's Westminster Abbey on Friday and pals say that Chelsy has been assisting him with his remarks because party loving Harry can't remember all the details of his and William's nights out.
A friend told MailOnline , "She's been helping with Harry's best man speech because she has a better memory of their nights out with William."
Meanwhile Harry - who recently returned from a charity trek to the Arctic - has hinted he may be eyeing up Mount Everest for his next adventure.
The third-in-line to the British throne who joined four injured serviceman to raise money for the foundation Walking With The Wounded - of which he is a patron said, "he might like to tackle their next challenge which involves conquering the world's highest mountain next May."
Speaking at a press conference to welcome them home, he said, "See you on the next one."
The four wounded men sustained their injuries while fighting in Afghanistan and the prince described them as "unbelievably inspirational".