Runoff is required by federal law to be contained onto the property under construction and cannot, for any reason, encroach upon adjoining properties. As a matter of fact, post-construction runoff isn't permitted by law to exceed pre-construction runoff. It has to be contained (piping, underground vaults, permanent sediment basins, etc.) and slowly released back into it's natural flow patterns.
If true, this isn't going to be an engineering issue. It will be a construction company negligence issue. I've seen it dozens of times. Contractors get lazy (or intentionally attempt to cut corners) by not installing proper erosion and sediment control measures prior to beginning construction. The inevitable happens and they attempt to blame anyone and everyone they can. It never works because engineering plans are scrutinized to the nth degree before construction permitting is issued.