I have a cousin who went out with a girl for about a year every week, she (cont. with details section)?

She was going out with another guy at the same time without telling the other guy. Even though the other guy was just friends, when one goes out with a girl for that length of time and its just one the 2 together every time, shouldn't the girl (for the principle of consideration and respect for him and the relationship ) tell him that she is or plans to go out with another guy?