Indeed, in Stanza XIII, he has the audacity to make a claim for the "truth." This, as the reader has come to expect at this stage, is only very brief. The only claim to truth is that the woman was indeed light. However, because of this very lightness, she claims not to have done any wrong. She disregards the feelings of the friend in favor of her own desires for life with the speaker. Her exclamation to "Never mind that youth" appears to echo the feelings of the speaker. The woman has done the speaker no harm, and he has not harmed her. Instead, together they have harmed the innocent friend and broken what friendship there might have been left for him and the speaker. "Never mind" here can therefore also be interpreted as "I don't care"; a truth that the speaker does not bother to address.

The claim to sincerity and truth throughout the poem is the very thing that reveals the speaker to be false and insincere. His initial claims to these qualities soon break down in the face of his friend's reaction and the final lines of the poem. For Robert Browning, broken friendships and hearts are much more interesting artistic material than truth and sincerity. Both the poet and the speaker then appear to use the friend's tragedy for their own gain.
Once again, to return to the question in the first stanza: the friend is to be pitied most, as both the speaker and the woman are "light" enough in their lack of loyalty to hurt him for the sake of a frivolous fling.