Advice?

Well I've been dating my boyfriend for over a month now and it's been amazing. We get along really well and he's my best friend now. I didn't think anything could go wrong but it did earlier today. His parents are out of town so he's staying at his friends house for a couple of days. His friend has a little sister just a year younger than my boyfriend, she has a crush on him and is a complete slut. I didn't think anything would happen between them but I was wrong about that. My boyfriend was alone with the sister for about an hour and she tried to mess around with him. She ended up getting in his pants, no not as in having sex together thank god, but my boyfriend told me he was so close to doing something with her but then he realized what was going on and stopped her. I realize technicaly my boyfriend didn't do anything wrong, I just don't like that he didn't react sooner than he did. He actually liked it. I feel betrayed because he wasn't careful around the sister and didn't try to prevent anything from happening, he was so in the moment it took him awhile to stop it from happening. I'm not entirely sure what I should do in this situation yet. Can you tell me if something like this has ever happened to you and what did you do?