This is so going to be solved by both falling in love with him, making both their hearts his. Makes more sense this way too, as it is a mermaid spell. Mermaid
flesh gives immortality (not reincarnation), but the little mermaid is a story about a mermaid gaining an afterlife through love (described as "an immortal soul" iirc). Imho the latter fits the description of the spell far more than the former.
iirc there was a plot point that he could make magic tools, so he would the second witch and the others would be the first. That way he can perform it twice.
A random journalist appears in a world at a mirror you have to look like the mom to pass through... in a garden owned by the mom... And transformation boots are missing... Yeah, this person is most definitely the mother, still hiding for unexplained reasons. And the way she explains the spell is likely a test for her daughters boyfriend.
edit:
@mekrabbat heh, true, the mc and cast is fairly stupid so that is quite possible. Assuming author is willing to do a genre-shift to keep the story true to their characters.