They might be lying or might be idiots and not realize defaults in that extension highlight some comments. They could just have it to do the bit that shows a comment on the bottom of the screen in a bigger font.
Other thoughts I've had on the weird Christina picnic:
What if Christina is Tim's alter ego or a code name? (I started typing this before
@Prosperity Piglet suggested the same thing more or less)
What if some Christina just canceled last minute on that picnic and they offered it to JennBo cheap?
What if Christina is Amanda's real name or a middle name?
I don't see Jenn being capable of being able to keep up the elaborate lie of having a new BFF. So the person is likely somehow real. But on the other hand maybe she is so jealous of Tim and Nick she had to get herself a new BFF either real or imaginary?