I gave my husband little "bits" he could use with our daughter when she was little, basically because his dad was not nice to his sisters at all and my husband loved his little girl but didn't exactly know how to go about "stuff". He might really be open to hearing some suggestion without EVER letting him know the diary business...he might inadvertantly let the cat out of the bag. He's just seeing her grow up and who among us ever know what to do with a teen, let alone a dad with his daughter. I'm sure his ears will perk up and he will soak in some much needed "ideas" about how to talk or things he can do with his daughter.
My daughter adores her dad...she's now 25 years old. He was always a pushover for her and still is. His little girl!