BCB: If your Dad has been packing a Ruger revolver for a number of years, going to a semi-auto may not be the best choice for him. You may wish to consider a lighter-weight snub-nose revolver instead - it would simply be 'scaling down' his current choice of defensive handgun.
In this regard there are a number of great choices, the S&W J Frames, and my personal choice is for an Airweight Bodyguard, Taurus has a number of lightweight DAO or DA/SA snubbies that would fit the bill and ther are probably others as well.
If Dad has been 'thinking' about a semi see what he would think of a lighter weight revolver. The 38 Spl, even in a lightweight revolver may be a bit easier on a 74 year old wrist but that caliber has been proven to work just as well as many of the others. Maybe some range time is in order for Dad, and with a number of choices. Just my 2 cents. Mikey.