Just be extremely supportive.
She's going to be extremely exhausted, sick, and irritated after the chemo, so if she snaps at you, cancels dates, etc., just let it slide. Don't baby her too much because she'll feel completely useless, so treat her as normally as possible, but still pay attention to see if she's having trouble.
And most importantly, keep in mind that lung cancer IS survivable.
My grandma had it and survived, and your girlfriend has the benefit of being young and strong.
Good luck to you, her, and her family!