Translation:Her grandfather is a friend of my grandfather.
I went wrong when wrote 'is friend' instead 'is a friend'. What's the difference?
Singular countable objects like friends need an article like a, an, or the.
I know it's been two months. In case you're still wondering about this or looking for feedback, I would say that for future reference, more information would be helpful. What were you doing? What was your suggestion? Copy-paste helps.