You cant transform into a mermaid. It just isn't possible. You can do spells to ENHANCE your beauty or make someone appear ugly to people, but these spells dont transform anyone physically, they just make you SEEM more beautiful in other peoples eyes or vice versa. So dont waste your time and effort, it wont work, period.
I was agree what Daisy_Chains and cem21 says.. Human cannot transformation.. Maybe you can find out and sneak at your friend home and see it.. That what she doing... If you see her not transform be a mermaid.. Probaly is she was lying at you.. Try to sneak at her home and find out to know its true or not..
As others have said, you cannot turn yourself into anything else by using spells and magic. You are a human, your friend is a human, and you cannot become anything else. Claiming that such spells work is considered role-playing and is against the rules of this site.