Not telling not telling
There is no normal ending of the situation at all, except when the girlfriend does not find out that you knew about the adventures of her faithful. It is not always possible to keep it under control. You will tell - an insult, you will not tell, and she will then learn that everyone around her knew, except her - an even greater insult. It would be good for the fools not to engage foreign people in their betrayal, if they themselves do not know how to live honestly, but unfortunately.