He Cheated!

So me and my boyfriend have been together for 3 years now and literally 2 hours ago his side hoe come to our house and confronts him for never telling me (the fiancé) about taking her on a date over two weeks ago. He claims it was only a date when we were arguing back then and we're thinking of taking a break but since we've worked tings out, he cut things off with her. But then she shows up calling him babe and threatening me so I'm just stuck, I don't know what to do... should I believe it was just a date and nothing else?