I noticed this same thing when I bought my tickets for my May trip. The great thing about it being only $1 more is that it gives you the freedom to choose any restaurant (not just a resort restaurant) for dinner on that extra day since you are only paying $1 to go into the park. It makes it much easier to plan your trip if, like us, you have a list of restaurants you really want to try and many of them are inside the parks.
If that extra park day would be on your departure day, it means you can go back into a park if only for a few hours before you have to leave for the airport. It's so nice to be able to do that for that one last stroll down Main Street...
So, if your friend likes to eat, try using the dining incentive to get her to commit to the 6-day ticket. If she just can't get enough of the parks, use the "last few hours in a park" argument.
If nothing else works, try arguing that you'll waste a lot of time standing in line to get the ticket upgraded when you get there, IF it's even possible to get the same deal. What if she waits until you get there and then discovers it costs more than $1 to upgrade?
And another thought... if your friend is agonizing over spending $1 on something that is such an obviously a great deal, is this someone you want to go on vacation with? I've traveled with someone like that and it was no fun! At dinner time all she wanted to do was find a McDonalds because she wanted to eat as cheaply as possible... and we were in Europe at the time!! Can you imagine going to Europe and then eating only at McDonalds? And the dollar was so strong against all European currencies that you could eat in a wonderful restaurant for next to nothing!! Sheesh!