I know this prayer by a German theologist, it's surprisingly well-known among Russian orthodox Christians.

You know, Yahia, my family member was diagnosed with cancer too... Just yesterday he got the test results, and you know what he said? "I'm glad I can pay for my sins here on Earth by suffering, I'd rather pay here than there, where it would be eternal" I was surprised by his strong faith and strength that is obviously given by God! Your piece has this spirit! The spirit of the power given to us by God in our most difficult moments.

I wish your family member to feel this support!