Is utilizing technical specifications from a previous project, with modifications, considered a diminishment of the right of the consulting office that prepared them, and is it forbidden for an employee to disclose this information without the permission of his employer?

If what you did was merely an imitation of previous specifications to benefit from them in designing your project's specifications, there is no harm in that, unless the owning institution prohibits access to its information for a legitimate and recognized purpose. In that case, your friend is not allowed to disclose it except with permission. However, if what you did was copying those specifications with minor modifications, it is not permissible except with the owner's permission, because their right is protected by Sharia (definition), and you must compensate them unless they waive their right, along with repenting to Allah.