Same thing happened with my pregnant mermaid yesterday!!
I also tried to get her a 2nd kelp, didn't work.
(She doesn't have mermaid scales, looks like a normal sim but has the needs of a mermaid)
But then a few sim days later she started to get the "been away from seawater too long" debuff again, this time she's not pregnant so I'm hoping she'll become normal again.
So maybe if you just keep her out of the sea for a while she'll go back to human.
**Edit**
She did not get back to normal after the 2'nd time either, I saw the "cure mermaid" action on her list, but the action erased itself before anything happened...
I just want her to stop being a mermaid already!!!
She doesn't have the "forever a mermaid" thing either. /=
