Yes, it is possible to prove fault in the case you describe, but would likely require--in addition to the injured person's own testimony about what he/she did and saw--1) medical testimony (e.g. from a medical examiner or other forensic physician) to review the autopsy and provide an explanation for what happened, the timing of injuries or heart damage vis-a-vis the accident, etc. and 2) other expert testimony, such as from an accident reconstruction specialist, who can take all the testimony and medical data and propose sound theories as to how the accident occured and who was at fault. So the answer is, yes, it's possible, but it may not not be easy and will likely not be cheap, because expert testimony is likely necessary.