Should my wife tell her Dad that she's not his daughter?

My wife found out Saturday through an ancestry.com DNA test that her dad is not her actual biological father. Her mother had a supposed one time incident with a man she found on Facebook through the names on the ancestry test. Her parents separated when she was 6. She wasn’t close with her dad over the years, but there was nothing ugly about it. Now she has been getting closer with him. She doesn’t want to tell him that he’s not her biological father, as that would hurt the relationship.

I told her she needs to tell him, because honesty is a building block of a relationship and that he’s still her father. If he finds out through the test that he took too (and didn’t put it together that she’s not his), then he will be devastated that she didn’t say anything. My question is, should she tell him or not?

I’ll support her decision either way, but I think honesty is the right thing. The right thing isn’t always the easy thing. I understand that her Dad, who raised her, will always be her true father to her.