If CBP told your mother that she should do AOS, then I can understand that you didn’t think anything of it. But that was kind of a strange action by the CBP officer, because they pretty much advised you something that is exactly the reason why people usually get denied entry for. Maybe they meant petitioning for an immigrant visa for your mother and not AOS, or maybe they actually let her enter enter knowing that she would file for an adjustment of status, I don’t know.