It's so strange they're so casual about what Ianto has done. Turning his back on Jack and having sex with her and going out with her, and there are no consequences. But when he sees Jack kissing her, it's Jack who feels sorry and apologises. Despite his earlier jealousy.
But the idea was marvellous. It's unique to see Lisa disrupting the already established relationship between Jack and Ianto in such a plasusible way.
I can't help thinking though that Ianto so ealily turned to her. Hm...