Hey, it's the conversation that should have happened before Momoka cheated. Rin never realized Momoka was in love and was operating as a lover replacement, and the moment Momoka makes her feelings somewhat clear, she runs into another girl's arms to fill the void... before Rin could react or comprehend the situation. All of Momoka's issues have been handled in isolation, and when Rin tries to step up, she runs off and cowers.
To be honest, I have no idea what I would do in Rin's position. I'd be worried this is what would happen any time they fought (meaning Momoka running off and making mistakes to cope with the pain). Either Momoka needs to show some major growth and trustworthiness, or they don't actually need to be together.