That's just utterly idiotic. Look at the two pieces of writing, they are the same, by the same person. A child could see that and they could also see in the first one that more than one person wrote part of it, the top part versus the bottom part. You are so far from grasping at straws that you can't even see the Dairy Queen on the horizon anymore.
Even if it is his signature, what crime is it to sign a yearbook? It means nothing. Nothing at all. It's the fact she added the date to make it appear to bolster her story. The date is crucial to her fairy tale to prove she quit the day after.
And by the way, no one signs a year book in December. She's toast.