By making aqd nikah between her and his son, or letting his wife breast feed her -if she has a new born child- full feeding 15 times.

If none of the two ways I mentioned is applicable, either because the man do not have a new born child, that his wife breast feed both babies, or he do not have a male boy son, to make nikah between them, then there is no way for the girl to be mahram for him, except by marrying her mother, and have full intercourse with her.