Just to clarify the legal situation, there are two routes whereby a new birth certificate can be issued.
If no father is named in the registration and on the original birth certificate, and the mother then marries the biological father, the birth must be re-registered with his name. If she lives in a relationship with the biological father but does not marry him, they can apply for his name to be added to the registration and apply for a new birth certificate.
As others have said, if the mother marries someone else who is not the biological father, they can apply to adopt the child (and both parents adopt, even the biological mother). When the adoption order is granted, a new adoption birth certificate is issued which replaces the original.
From what the OP has said it seems she was involved in their lives at the time and would have known if this man had been involved in adopting her granddaughter. As she is certain this did not happen it seems more likely that the registration was amended to include the new partner’s name. If he is not the biological father, this was indeed a lie and a false registration. Whatever the motives, it is unfair to a child to register their birth with false information. Hope that helps.