All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Reciting Al-Basmalah(i.e saying Bismillaaahi Ar-Rahmaan Ar-Raheem)while making Wudu(Ablution) is a recommended act of Sunnah, and whosoever leaves it, intentionally or forgetfully, should recite it during Wudu`.
Moreover, Al-Basmalah is not an obligation since it is an act of Sunnah. In addition, Muslim scholars are of the view that the Prophet's Hadith in which he said, "The prayer of a person who does not perform ablution is not valid, and the ablution of a person who does not mention the name of Allah (in the beginning) is not valid.", indicates that reciting Al-Basmalah during Wudu` is preferable, but not doing so won't affect the validity of Wudu`.
Therefore, not reciting Al-Basmalah while making Wudu won't render it invalid, and the person who didn`t recite it is on the safe side, but he has missed the reward of an act of Sunnah. And Allah knows best.