I want to know what evidences are good enough to prove that a heart operation was unsuccessful?

The medical report(s) from the consulting doctors will provide evidence of medical malpractice.  Medical malpractice is based on negligence.  Negligence is the failure to exercise due care (that degree of care in this case that a reasonable cardiac surgeon in the community would have exercised under the same or similar circumstances) to prevent foreseeable harm.

It would be advisable to speak with a medical malpractice attorney.

Prior to filing a lawsuit for negligence against the surgeon and hospital where the surgery occurred, it may be possible to settle the case with the insurance carriers for the surgeon and hospital.  Your father's claim would consist of the medical bills, medical reports, and documentation of any wage loss.  Compensation for the medical bills is straight reimbursement.  Compensation for wage loss is straight reimbursement.  The medical reports will document the nature and extent of the injury (negligent surgery) and will be used to determine compensation for pain and suffering.  Compensation for pain and suffering is an amount in addition to the medical bills.

Since the original medical reports don't document the negligent surgery, the insurance carriers for the hospital and surgeon might deny the claim.  Your father's claim should include the medical reports from the consulting doctors which document the negligent surgery.

If the claim is denied or if it is not denied, but your father is dissatisfied with settlement offers from the insurance carriers from both the surgeon and hospital, your father can reject the settlement offers and file a lawsuit for negligence against the surgeon and hospital.  If the case is settled with both the surgeon and hospital, no lawsuit is filed.  If the case is only settled with one of these parties, the lawsuit would only name the remaining party as a defendant.  If the case is not settled with either party, the lawsuit would name both the surgeon and hospital as defendants.

The lawsuit must be filed prior to the expiration of the applicable statute of limitations or your father will lose his rights forever in the matter.

First, speak with an experienced medical malpractice attorney, bringing whatever information, etc. you have; the attorney can help you decide if the case is worth pursuing, based on the likelihood of liability and also the damages (compensation) you may collect.

Note that medical malpractice cases can be among the most expensive ones to pursue; to prove them, you will need medical expert testimony--e.g. the testimony of at least one, possibly more, doctors or other medical experts--which can cost thousands of dollars (experts don't work cheap). You'll also probably need a lot of tests (e.g. EKGs, possibly CAT scans or MRIs, etc.), which also do not come cheap. In short, to prove medical negligence (which is what malpractice is), you need medical expertise.