Amy knew the first place to look was the magic shop itself. Even if the change is permanent, there may be something that can at least temporary return Lina to human form.
She entered the shop to see the wizardly clerk thumbing through the magic book before looking up at her. "Can I help you with something?" He asked.
"You damn well better!" Amy shouted. "My friend is stuck as a mermaid, and I need a way to turn her back!"
"The receipt says that all changes are permanent."
"I know what it says, but if there's a thing that can make a human into a mermaid, there must be something that works the other way, right? So tell me what I need; a spell, another magical item, true love's kiss, out with it!" Amy was borderline hysterical at this point.
"Calm down child, you wouldn't want to do something you'll regret; nor would I, likewise."
Amy gritted her teeth, closed her eyes, took a deep breath and exhaled. Much calmer now, she said, "I'm sorry, it's just that... I've known Lina since we were in Kindergarten, we're best friends. It just pains me to think I caused such a thing to happen to her just because I dragged her to this godforsaken store."
"I can see that you care for her deeply, and you'll stop at nothing to make this right."