Strange plot twist

Rating: 7/10 (1 December 2019)

Towards the end, after Marta and Ransom have had their lame car chase, Ransom gets arrested by the police, and Marta goes to the abandoned store front and finds the almost dead housekeeper. It is shown that Ransom has given the housekeeper a lethal dosage of Morphine, which at an earlier point is said to be deadly in 10 minutes. So, the housekeeper should definitely have been dead by the time Marta gets there, but she is still somehow alive? Can someone explain this to me? When did Ransom kill the housekeeper? How did the housekeeper manage to stay alive so long?