Interesting. When my brother was 30, his 38 year-old wife's 18 year-old daughter gave birth to a baby boy, making my brother a 30 year-old grandfather. Many jokes were made. ;)
Sorry, but that doesn't make him a grandfather anymore than marrying the woman in the first place made him the daughter's father. My ex may remarry, but that only makes that guy her new husband, not my son's father, and nobody feels that more strongly than my son.
The whole premise to all this is that the titles given to a person involved in some woman's life are hers to bestow with no concern for biological or social reality. Too many women think they can define and re-define these things according to their latest romance.