He doesn't reply until he's done describing the spell, and then they're both invisible—except they can perceive (not quite see) each other—and intangible—except they don't fall through the ground and can hold things if they want—and inaudible—except they can still talk to each other.

"Humans don't really have the option. I'm still super curious about what you do instead, though."

"I just talk."

"That doesn't really allow me to make any new predictions."

"I don't know what you want me to say! It seems perfectly straightforward to me. Anyway this not seeing thing we're doing now is - weird."

"Yeah, it is. Useful, though. And you'll be able to stroll through right in front of whichever fairies may be guarding the tree and they'll be none the wiser."

"Thank you."

And she steps through.

She comes back with a branch and some books.

"Was no one guarding it? Or were there branches inside your tree?"

"I can move my tree however I want so I moved a branch inside."

"Well that's great, then. Did you want to grab any other books? My backpack's bigger on the inside, and I can make it temporarily weightless for that to not be a problem either."

"These are all my books. I didn't have time to accumulate many."

"Oh. ...I hate Thorn very, very much."

Nod.

"I don't suppose you'd let me come with you and borrow your books?—or, I guess, point me to a library, if there is one."

"There are libraries but it's probably not safe for a mortal to go to one. You can borrow my books."

"Oh," he says, brightening up considerably. "Thank you!"

"You're welcome."

"Do you have an introductory one?"

She offers him a blue book.

He takes it, beaming. "You're sure you won't need it?"

"I'm past that. You can keep that one."

He will not offer to hug her, she is clearly still not comfortable with physical contact. Instead he will just keep grinning and repeat, "Thank you! You're great."

"How long do you think I'll take to get through this one?"

"Depends how good you are."

"Is skill at this its own thing or is it correlated with, like, attention, or general intelligence, or anything like that?"