MTN could be the reason my Engagement is breaking ???

My fiance lost her Sim card and within a couple of days someone by the name of Zanele is now using her Sim card, she is even trying to change her whatsapp. She has give numerous guys the number and every day or two there is a new guy whatsapping my fiance asking the meet her. We called MTN and we were told we can only block her phone and not the cellphone number, do I need to mention that we use our cellphone as ever point of contact most especially our banking details? My fiancé' financial detail are in the position of a strange and MTN does give a damn about helping us