In this scenario, a duty of care neither existed between Monica and Rachel during the course of their criminal enterprise nor during the course of their subsequent flight or escape, because the defence of ex turpi causa non oritur actio applies. This legal principle translates to "no action arises from a dishonourable cause," meaning that the courts will not assist a claimant who bases their cause of action on their own illegal act. This is rooted in public policy, aiming to prevent the law from being used to facilitate or profit from illegal activities. The application of the defence depends on a consideration of all the facts, and the courts generally do not recognise a duty of care owed by one participant in a crime to another for acts done in the course of its commission. 

Key Point: The principle of ex turpi causa non oritur actio serves as a defence in tort law, denying a claim that arises out of illegal activity. This was upheld in the lead case Ashton v Turner [1981] QB 137, where the court refused to recognise a duty of care between participants in a joint illegal enterprise. This case illustrates how public policy considerations can override other legal principles in order to prevent the courts from being used to resolve disputes arising from criminal conduct.