I would think that first you have to have a reason to gather the evidence, a reason to believe there was willful neglect or malpractice.
The key questions that need to be answered immediately are, what symptom leads you to believe there is something to investigate? Has any free internet research been conducted that suggests malpractice? Has another doctor been consulted?
There may in fact been some serious malpractice, but the question, as presented, showed no curiosity about what happened. I will give the benefit of the doubt to someone who is in the middle of a personal tragedy and may not express himself to his best advantage.
"willful neglect or malpractice"
The standard of proof for negligence is not "wilfull"
No one commits willful negligence. That is called either recklesness or intentional, which could be a crime.
Negligence often involves deviating from an accepted standard of care by similar professionals in the same circumstance.
Believe it or not, med mal cases are very hard to prove.