Neglience - anyone well versed willing to offer some guidance?

Q: There is a road-traffic accident due to the careless driving of David. The victim, Michelle, suffers facial injuries. After the crash, Michelle is in a state of shock so Steven, another driver, gives Michelle an energy drink to try and clam her down. However, Michelle has diabetes, becomes unwell and subsequently goes to hospital. The doctor, Dr Asif, gives her medication to which she has an allergic reaction and must say in hospital for a week longer.

I understand the basic tenets of negligence; however, I’m having difficulties ascribing liability here.

I'm aware that David is liable in negligence for the original injuries. However, what's confusing me is the extent to which he is liable. Would David be liable with regards to the allergic reaction and the prolonged stay in hospital? It is foreseeable that hospital would arise after a RTA, therefore could he be held liable? The case law is quite convoluted on this with some cases saying that a defendant is liable for the full extent of the injuries?Could the chain of causation be broken by Steven? i.e. by exacerbating Michelle's diabetes by providing her with a drink and causing her to go to hospital?

Does material contribution come into play? i.e. could Stephen's actions materially contributed to the injury?

Can anyone offer me some guidance on this? Thank you.