Sounds like a communication issue between them rather than a betrayal as it could have been solved had they discussed their intentions and desires first. I can forgive them but I would be concerned that the person doesn't know me enough to know I wouldn't want that... so a talk would be in order.
Because the road to hell is paved with good intentions.
Also, I'm glad I never had that kind of shitty betrayal in my life.
Hope that it will never happen to me.