Is your girlfriends permit for the Whitney Main Trail or for a westside approach? Makes a big difference, if it's for the Main trail then you don't have apermit to come from anywhere else. But, if you want to go by way of Bighorn Plateau, the shortest route to get to Bighorn is Shepherd Pass. 2 days from the Shepherd Pass trailhead should put you at Bighorn, and you can be at the Whitney summit in a day from there, with another day to get to the Portal - although you could do Bighorn to the portal by way of the summit in one long day if you are in shape for it.
Acclimatization depends on you and how you handle altitude. A night at 7 or 8,000 feet before you start would be a good idea.