85 year old father has acute kidney failure directly following colonscopy. Is a biopsy the only way to prove it is malpractice?

My Father had kidney levels of 1.6 prior to a colonoscopy and should never have taken Fleet Phospho-soda prescribed by his doctor, who SHOULD have known of the serious danger in his particular case. Immediately after the colonoscopy his kidney levels were 2.7+. What can he do without having a dangerous and potentially more harmful biopsy to prove malpractice? Please advise. He can use some help.