If Irina and Pete dating pre-date Kate and Pete dating I don't see it as throwing it back in her face, or stabbing her in the back, just reconnecting with an old ex. It's not like Kate introduced her to Pete or anything. It's not the classiest behaviour I suppose, but then nothing to do with Pete is, and I don't find it a betrayal (though none of us know what goes on behind closed doors or anything), especially as Kate has publically (thankfully) moved on and Kate is hardly a good girl when it comes to relationships.