I don’t know if I should be worried about anything or not with her still having his last name. I just can’t help but feel like she might have done something that he doesn’t know about and that worries me for when we get married. If his daughter had his last name then I can kinda understand why she would but their daughter doesn’t even have his last name so it just makes it all weird to me. It could be absolutely nothing at all and just my mind over thinking things like always 
. I would appreciate any advice I can get 
Were they married? If yes, then it is legally her last name, regardless of what name her child has. I kept my married last name until I married again. There should be nothing to worry about unless you just want to.