There's no doubt step through frames are comfortable and handy to mount and dismount, and offer significant advantages especially to older riders who are a bit less flexible in the hip, or less confident riders, or people with kiddie seats or high luggage on the rear rack. The disadvantage is that the design is inherently less strong than the traditional "diamond frame" with a cross bar (top tube), and manufacturers may compensate for that by using sturdier materials (fatter tubing) that makes the bike a bit heavier.

But your Dad should be the one to decide. Take him shopping and have him try stuff out.