Episode 5.5 [Rewatch]

[Tea Lady]

Harry knew. When Adam collected him from the police station he told him the plan and that Ruth had sacrificed herself. Yes there is a missing scene there, when Harry and Adam probably fight about the decision, but at that point the decision is already made. Ruth has threatened that woman with a gun and the plan is already in its final stages. There is little Harry can do now.

When the call comes through to his office, Harry has to act that he is devastated to the rest of the team. Don't forget that Ros is not exactly to be trusted. He knows the body is not Ruth.

Also Ruth told Adam not to tell Harry where she is because there was a chance he could talk her out it.