To each his own, as Hammer said.
I would tell in a way that would come across as concern.
Dad? Are you and _____ alright?
Ok, I was concerned as she changed her fb page from married to blank.
It could bring the two of you closer.
However! You know your relationship with your dad better than anybody, and your gut probably told you from the get-go what to do here.
As for her-she's a grown woman that should know better than to put out there publicly such a change and not expect questions. It's fb for gosh sakes.