First, let me say, I am sorry to hear about your loss. May your grandmother rest in paradise.
As, for the question at hand.. I believe not all things are black and white. Some things just arent this or that. I think emotional cheating is far worse than say, a one time hook up and I dont think that "once a cheater, always a cheater". We are human. We make mistakes. The deciding factor is if we learn from those mistakes and how they change us as a person. I could forgive my husband for say, having a drunken night and fucking up. What I couldnt forgive, is an ongoing emotional connection/affair. Im not one of those big believers that sex is sacred and is only magical when it is shared with someone you love.. Sex is sex. And its great but to me, its not the most valuable part of a relationship. I would look at the entire situation and make a choice based off of the person and WHY it happened. Rebuilding trust is difficult but you have to weigh if one mistake is worth throwing everything away for or not.