Would you tell your friend if you thought their significant other was ugly?

No.
I had a friend in school who considered his girlfriend to be the most beautiful girl he had ever seen. She was quite average looking at best.
However, in getting to know her a little, she was one of the nicest people I’ve ever met. I found myself smiling a bit every time I saw her....and I began to understand why my friend thought she was so beautiful (because in a way, she was).