Dear ....(имя друга, Andrew например)
I decided to write a letter to you, to warn you.
You asking me to go with you in the movies. I'm sorry to say this, but I got sick and can't go with you. Thank you for the invitation, I am very pleased.
I hope that I will recover your invitation will remain in force, and we watch a movie together.
Offer to go to the movie " Dubrovsky", I think he will be an extraordinary film adaptation.
With best wishes,
Vika.