My hubby plucked up the courage to do it so she's now gone. He said it was very peaceful, her side fins just slowly stopped waving and then she was gone - took seconds.
It was definitely not a worm, he said he thought he could see a piece of daphnia in there and so is convinced it was her insides.
Just some daily water changes.
Don't need a med it just happens with livebearers now and then.
Keep a look out for the internal parasite signs though.
Good luck.