The company that requested the penetration test is responsible for providing the correct and accurate network scope for the test. The network scope defines the boundaries and limitations of the test, such as which IP addresses, domains, systems, or networks are in scope or out of scope. If the company provided an incorrect network scope that included an IP address that belongs to a third party, then it is responsible for this mistake.
The penetration testing company, the target host's owner, the penetration tester, and the subcontractor supporting the test are not responsible for this mistake, as they relied on the network scope provided by the company that requested the penetration test.