James can be further pardoned by the fact that he is responding to a lady's tears: Isabella is, at the moment that he joins the urging, pretending to cry.

Isabella, in the meanwhile, had applied her handkerchief to her eyes; and Morland, miserable at such a sight, could not help saying, “Nay, Catherine. I think you cannot stand out any longer now. The sacrifice is not much; and to oblige such a friend — I shall think you quite unkind, if you still refuse.”

I am sure that, irregardless of his affection HERE, he has been taught to protect the ladies, and see to their comfort. Is it any wonder that he chooses the one duty that is more visible (for Catherine is neither answering, nor appearing upset), rather than the one that will upset Isabella further?

However, seeing as how Isabella's sights are obviously set on young Mr. Morland, I suspect that he had another recourse: had he only known it, I would say that he could have eased the whole issue, and proven his hold on Isabella (at least, to his own satisfaction). If James had said, "Why, Isabella, does this mean that any engagement you have with me, or any other of your friends, would give way, should a better scheme occur to you? You are not considering my sister's feelings, nor propriety. We can easily manage something to amuse us tomorrow, and change the day of our drive to the day after, I dare say." Isabella would have acquiesced immediately, and John Thorpe could not have stood long against that confederacy.