I don't know either party either & all we have to go on are newspaper articles but there does seem to be reports that there were big rows between the pair of them about Sean's (probably understandable) jealousy over Georgia's closeness with Giovanni, so I don't think it was quite as black & white as you are making out, ie Georgia sudeenly falling out of her relationship with Sean & into one with Giovanni while 'poor Sean was unaware there was a problem'!!
Relationships are complicated, especially when you're young & sometimes what you thought was everything can soon turn out not quite so important as you thought if something even more inviting comes along!! I suspect that's what happened with Georgia & you can either point the finger, call her cruel, slapper, tart, whatever you choose, or you can say why shouldn't she move on with her life in her own way & let Sean move on with his! At least she wasn't sneaking around seeing both together!!
I hope Sean finds a girl he loves who loves him back just as much & he gets his happy ever after!!
I hope exactly the same for Georgia with her relationship, if Gio is the one then fantastic, if not then I hope she gets as much fun & happiness from life as she can possibly get until she does find Mr Right!!