I have seen this happen quite a few times here in Doha (the bleeding I mean, not incompetent vet). Labs can have genetic problems due to overbreeding and clotting disorders are very common, especially in the pet shop King Charles and Labs here. Why not start by asking him or her for a written statement along with a copy of the patient/client notes? I know you are angry but you first of all need to establish a cause of death. Post mortem and blood profile/tox screen etc are the only way to prove incompetence (after all, rat poison in small quantities can cause death by bleeding) and the first step should always be to gather information and try to resolve this at the clinic level.

If you don't gather evidence and try to prove that you have made reasonable effort to solve the problem, most legal systems will not be able to support you.

I hope your friend gets the answers they need.