First of all a "Bad Marriage" is never good for the children or the parents, That being said I think in most cases whats good for the parents is good for the children. In my opinion couples should never stay married just to make family happy. After all marriage is about 2 people's happiness together, not because it may or may not make family members happy. If you truly care for this persons family then cut off this relationship with said person and avoid a disaster in the future.