Tell her exactly what her unhinged and abusive parents told you. Those control freaks are demanding you sabotage birth control methods to give them grandchildren. These are the kind of people that should never be grandparents as they are unhinged, selfish, manipulative, deceptive lunatics that would pull all kinds of abuse outrageous stunts. Do not keep this secret. Never stay in their home. Tell your gf every detail of that conversation. She needs to know how far her awful parents are willing to go to destroy her life, sabotage her future, violate her boundaries and body autonomy and disregard her feelings. These abusers see their daughter as a baby incubator, not as a person who gets to make her own decisions about her life.