Execution petitions and attachment of property

A client of mine bought a property in 2017 which was in joint ownership of a mother father and son. The son was a judgement debtor in a company and didn't disclose the same and neither was any pending litigation revealed after due diligence as the litigation was against the company he was a director off. Now my client has received summons from court regarding the validity of the sale deed. What steps can be taken in such a case?