I heard this phrase a lot after being diagnosed with ALS. I was grateful, but also overwhelmed.
My advice if someone you know receives life-changing news: offer to do something specific. Rather than saying “I can’t imagine,” really think about how their world has changed. Lots of doctor’s appointments? Uber gift cards. Your kids swim together? Volunteer to drive to practice. Homebound? Spend an afternoon helping so the caregiver gets a break.
If you don’t know the person well, sending a card works wonders: “Just wanted to let you know I’m thinking of you, but no need to respond.”