If the trip is in a reasonably dry part of the year 28 mm slicks are fine IMO. They might even be my first choice if I knew it would be reasonably dry. It can get pretty sloppy at times though and during wet times a mountain bike can be pretty nice there.

If it is a one time thing consider renting or borrowing a bike with fatter tires. If it will/may be wet and if she will continue to use a fat tire bike regularly it may be time for a +1.