[Diarmuid isn't exactly sure how to respond. There is no reason Saber wouldn't be safe with her Master and his people, so leaving her now, as requested, would be okay. However, she seems to want him to stay and that makes him less willing to leave even if that means that it might take longer to get her back to normal.

Besides, he knows far too well that as soon as he returns to his Master and reports to him about what has happened, his ability to protect her will be gone. In fact, his Master might well order him to come back here and attempt to kill her while she is so weakened. It is why, in the end, he swings toward remaining here as long as possible. Ideally, until she is back to normal.]

With all do respect, it might be in your best interests for me to remain. At the moment, my orders are to aid you and Saber however I can. However, as soon as I return to my Master and report what has happened, I have no doubt those orders will change, and I will be forced to become your enemy. Would it not be wiser to have me remain so that you have one less concern to worry about while attempting to break Caster's spell?