Marrying those people is permissible. Everybody is entitled to choose his/her life partner in the way that he/she sees suitable whether they were born from a legitimate marriage or not.The father of the girl that is approached for marriage should choose what is best for his daughter. Defining what is best for her depends on the particular circumstances, social values and maybe other factors. It is possible that the father may see marrying his daughter to a person born out of wedlock to be against her best interests.