You should tell her ASAP.
Can you imagine how she would react if she found out some other way?
He simply has to tell her, there is nothing he can do to stop her reacting the way she will. That is out of his control.
However I would suggest he reassures her that his relationship with his daughter will not change and make her feel part of the whole process so she is not alienated.
Without knowing any more I am limited by what further advice I can offer.