The spell will simply push the other person away. It is not meant to harm anyone physically, but that spell doesn’t prevent accidents or illnesses from coming your way once you cast it. Knowing what spell to use can help separate people for good, meaning there are no feelings left.
However, if you are thinking of stirring the waters completely, you should consider black for separation. This is because dark magic entails using souls, so there is no doubt that spell casting will be an unpleasant experience. This means that someone will definitely get hurt by the spell. The only issue is that this can be several people rather than the target. The kids in the marriage might get hurt, the target and even their extended family members might fall victim to the unpleasantries of black magic separation spells.
https://ceramicinspirations.co.uk/articles/ending_marriages_with_separation_spells.html