Lost domain. How to untie it from the old site?

Question

CMS

Greetings!
I support the client's website. The site was made by the studio and registered a new domain. The client also had an old website and an old domain.
A 301 redirect was made from the old domain to the new one. And then, one day, the redirect falls off the old site.
I note that the old site was hosted by a private developer. Those. no access to this old site.
My client did not pay for hosting and domains, but entrusted them to other people. So now I only have access to the new hosting and domain.
People involved in payment say that the developer of the first site was involved in everything. This person says that he deleted both the site and access to the old domain.
I ordered an extract of the domain history and as it turned out, the paid period had recently ended. And a week later the domain was re-registered again. That is, now it is busy, but remains attached to the old site.
As a result.
There are two sites vetgospital.ru and 323-500.ru on the network with almost identical content. Both are valid and issued by a search engine.
What do you advise? Ideally, the old site should be deleted or unlinked from the domain. But how?

Answer 1

Try to negotiate with the new domain owner. In the current situation, search engines are more likely to affinate sites with repetitive content.