Road Bike - wheel set and tyres suggestions?

You can probably go to a 700 road bike set if the bike has 700s or 26s on it now. It will make a real difference compared to the weight and rolling resistance Of a 2.25 or 2,30 off road tire. If it has shocks maybe swap them out for a good light fork at the same time. If shes not using the gears now why not get a single speed hub and save yourself a few bucks and lighten the bike considerably. If not you will need the derailleur and shifter that matches your wheel. .