It is right of the child to have this milk, a husband should not engage in such activity on purpose because not only is it medically unfit for the person but is also usurpation of rights of the infant. However, this does not effect the relationship between husband and wife.
Regarding milk kinship, it must be kept in mind that such a relationship is not formed by some chance episode of suckling a child. The words of the Quran stated in this context clearly testify that this relationship is established only with the full intent of those involved. In other words, an accidental happening does not establish this relationship; it only comes into being after it is planned and is well thought of. Hence, it is absolutely certain that for fosterage it is essential that the child be in the suckling age and that the matter be a planned one and not an accidental one. (For Details: Discussion 29875).