No witness is needed. Even the wife’s presence is not necessary. That is why people need to stop behaving like Bollywood heroes and heroines and understand that on the day of judgement it is Allah swt that will be judging us not Shahrukh Khan or Aishwarya Rai.
JJ, you started another thread like this…I have posted there as well.
Tell the couple you are talking about to consult an Imam, Mufti, Alim as soon as possible. I don’t think GS is the right place. 