Your friend has a right to have his prior baptism attempt examined for validity and then, if the validity is seriously doubted, he should be conditionally baptized. Here is the applicable text from the Code of Canon Law:
§1. If there is a doubt whether a person has been baptized or whether baptism was conferred validly and the doubt remains after a serious investigation, baptism is to be conferred conditionally.
§2. Those baptized in a non-Catholic ecclesial community must not be baptized conditionally unless, after an examination of the matter and the form of the words used in the conferral of baptism and a consideration of the intention of the baptized adult and the minister of the baptism, a serious reason exists to doubt the validity of the baptism.
§3. If in the cases mentioned in §§1 and 2 the conferral or validity of the baptism remains doubtful, baptism is not to be conferred until after the doctrine of the sacrament of baptism is explained to the person to be baptized, if an adult, and the reasons of the doubtful validity of the baptism are explained to the person or, in the case of an infant, to the parents. (CIC 869)