It didn't need to go anywhere. What more should be said? She's a friend of Rachel's who isn't so close to her anymore because her parents are acting all creepy and psycho. Rachel says a few little "boo hoos", vows to fight the evil heathens, rabble rabble rabble, Rachel now has a more concrete personal motivation against the slugs.
Bada bing bada boom, now we can move onto the effects of the war on Tobias, Cassie, and Marco in succession. Book 2 did what it needed to. Up and above that, Melissa was just another background character affected by the invasion.