The only problem with the Agatha Cristie games, though, is that they are rated "T" - one for "Mild Violence," and the other for "Mild Violence, Drug Reference, and Mild Language." Even if it wasn't too bad, Dad would probably have to play it some by himself or do some serious research.
Mild Violence: Both games involve murders, though with an Agatha Christie murder they are guaranteed to be very genteel. I *think* all you see are crumpled figures on the ground, though in Orient Express there might be a still drawing of a gun discharging.
Drug Reference: In general, this is very likely to be the existence in the game of wine bottle, cigarette cases, and such. It is not impossible that one of the characters in the game is mentioned in passing dialogue as having an addiction of some sort, but I can't remember fully.
Mild Language: Par for the Christie canon. Expect to hear "hell" and "damn(ed)" without reference to God, and I do not believe there is any word more vulgar. Perhaps a reference to someone being a bastard (the out of wedlock kind).
Again, it's been a while since I played both games, but even if I'm misremembering some of the specifics, that is the general level of the adult references in the games.