It is not like someone with sz will always have children with sz but it may show up sooner or later in someone directly related. The environmental factors that trigger it seem more of a direct cause than genetics at times.
To answer the question, yes, if the person has sz symptoms, it is better to tell the person than for the person to learn later. It will just cause more problems if you don't. It is kind of sad that there is so much stigma but that's the way it is.