Ask yourself does she consider it cheating? There is your answer, regardless of how the story turns out Bella thinks she is cheating on her husband. Doesn't matter if she is or isn't, whether he is dead, MIA, hidden in the room in a coma, hanging from the rafters...... pick your theory. The fact that Bella thinks she is cheating on her husband is all the answer we need. So no I can't see her cheating on her husband and betraying her dead best friends little sister and giving a flying fluck if the MC is double dipping. Sorry just can't see it.