Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Dying hair with black colour is not permitted according to the most correct opinion of Muslim scholars. Abu Dawud reported in his book Sunan that the Prophet (Sallallahu Alaihi wa Sallam), when he saw the grey hair of Quhafah, father of Abu Bakr, he ordered him to change this (the colour of his hair) and avoid the black colour. (Muslim)
Therefore, your friend should not dye his hair with black colour; instead he may use any other dye to change the colour as long as there is no imitation of dissolute persons.
However, if the purpose of dying is to cheat the fiancée or her guardian then it is forbidden to change regardless of the colour, since the Prophet (Sallallahu Alaihi wa Sallam) said: "Whoever cheats is not one of us."
Allah knows best.