it is very rare, to my knowledge

that a boss wrote a paper without including his/her trainees who have work on the project. If the person is a research assistant, it will depend on the boss. However, your relative may lose fist-authorship, if he does not write. If I were him/her, I would talk to the boss and say i will write the ms, even leaving the lab. I would then try to finish it asap. It is good for both sides.