I would say that under DSE a manager does not have any specific responsibilities to his staff under DSE (or under any other pieces of H&S law). The responsibilities lie with the employer, who can of course some delegate the job of providing H&S to his managers but the ultimate responsibility stays with the employer.
A manager like any employee is expected to cooperate with their employer and to follow the employer’s systems and to report any short comings in the systems to the best of his abilities. So if you see that an employee is struggling and you think it is because their workstation set-up is inadequate then you need to report it. That’s it.