She doesn’t have to, but wouldn’t it be nice if she did? 

It’s easy to tell a kid in Elementary school to sit next to the kid who nobody else is sitting with to help make them feel included. Little kids are much more likely to actually take that advice because they are less self-conscious and easier to persuade than high schoolers. Once you’re in high school, you’re more likely to have the attitude that if someone is sitting alone at lunch, there’s probably a good reason for that, and avoid that person accordingly. Whether it’s right or not, we’re socially conditioned to fear people who don’t have any friends. 

While a teenager might not sit next to a stranger at lunch, they might feel as if they should include a lone wolf if they’re the child of a family friend, an acquaintance from an extracurricular activity, or a family member. If they have some base-level familiarity with a person and have no reason not to trust them, they should at least consider extending an olive branch to someone they care about who is genuinely struggling. No rule says they have to, but it’s the kind thing to do.