Do I tell her I'm an athiest?

I have a friend, near and dear for the past 20 years. She is a fervent believer. She is also very kind to all who cross her path and is much loved by all. In the past I have never had to let her know my position on god, but lately he is in the answer to ANY question I ask, and since I have become ill, she tells me to pray. As lovely as she is, I know her well enough to say that if I tell her I am a staunch atheist she will be disappointed to the extent that it will taint our friendship forever. PLEASE DON'T SAY THAT THEREFORE SHE IS NOT A FRIEND WORTH HAVING because she has a golden heart and we have been through much together. Do I continue "playing along" or do I tell her the truth?