I looked at that one website, and it's just a recommendation that they be in a belt positioning booster until 4'9" and 80 lbs. That's the law I believe in some states, but the law in Texas states this:
"Texas child passenger safety laws require children younger than five years of age and less than 36 inches in height to ride in an approved car seat or booster seat. Additionally, Texas law states that during the operation of the vehicle, the child must be properly secured in a child passenger safety car seat system according to the instructions of the manufacturer of the safety seat system. Children under age 17 must be buckled into the vehicle with a seat belt. This law changed on Sept. 1, 2005. The new law increased the age requirement for child safety seats from four to five."
http://babyproducts.about.com/od/statecarseatlaws/qt/texa...
So, in the front seat, she should be ok as long as you can turn off the air bag. I don't know your type of car, but maybe it would be safer if she rode in the back in a booster between the 2 car seats? I'm not sure what would be safer, no car seat in the backseat or a car seat in the front.