Pretty normal I would say. As a matter of fact, I only had AVR (done in just a few hours) and my family (husband and kids all in their 20's) still tell jokes about some of the silly things I said or came up with just post surgery (and I'm remembering all the way to several weeks out). I remember, too, getting sentences jumbled and thoughts spit out that were not quite what I hoped for for several months. They give us SO many drugs.... it's amazing...... elephant tranquilizers! well, almost. It takes the body a good 6-12 months to process all that stuff out of the system. You will not have to wait that long for your father to be mentally right again. He should be very close to normal soon.
If time passes and you feel in your gut that things are not right, then by all means, follow up with the surgeons and whatever counsel they can give you.
But for now, just be patient and smile with him!
AVR 4/18/06, age 52. Bovine tissue. CE Perimount RSR Model 2800. 23 mm. BAV diagnosed age 27(1981). Moderate stenosis (1.0 cm) 9/03; severe stenosis (.7cm) 12/05, Aortic measure at surgery 4/06, critical (.53 cm)