Were he asked, Diarmuid would say without a doubt he believes that Lancelot has the strength to do such guiding. However, no one can really know for sure how they will react until the time comes and they are standing in the moment. If he was asked to do what he is doing now back home in the war before all the changes that happened to him here? Diarmuid very likely would not have had the strength to do it.

Now, he doesn't think twice about it.

"Thank you and don't worry. I will take your warnings to heart. In the end, if he does hurt me, I know there are those who will make him pay for doing so and...." he laughs, "...then you can tell me, I told you so. Until then, I am going to do everything in my power to make sure when that moment of urgency comes, he does the right thing and doesn't just follow his instincts."