This:

Given your wife's weight 28's will be already be overkill for road use! They are quite capable of handling a typical unpaved rails to trails type of trail so if that is the worst sort of trail you envision riding the 28's will be great all around tires. If you know up front that she will need some specialized tires to handle the terrain you intend to ride then by all means get her the tires she needs. Otherwise the 28's will do quite well until she decides which tires she really wants based on the type of riding she ends up doing. I've got 100 lbs on your wife and I ride 38's on the local roads and paved/unpaved rail/trail conversions but the 28's that came with my bike served me very well on the same trails until I decided what tires I really wanted. Fatter tires are making a comeback, the size uptick in the newer models is probably just a reflection of this rather than an underlying need.