He's silly and sentimental and human, but here she is asking for his advice. And, surprisingly, planning to follow it.

"Glad you think so."

He won't tell her it's the right thing to do (because he can't be sure of it) or the grown up thing to do (because based on what he's heard, grown up trolls make shitty role models), but he feels something is needed here. He doesn't want regrets if she leaves again. So he blurts the next thing that comes to mind:

"I'm proud of you. No matter what happens after we leave here."