My best friend just found out his g/f cheated on him, why is he still with her?

My best friend and this girl have been dating for a few months and he found out that she cheated on him with and had sex with one of his best friends! I was so shocked that he is still with her. He claims he doesn’t feel like looking for another g/f so he’s gonna stay with her. Do you think that is the real reason? Do you think he won’t leave her because he’s insecure? What is other reasons why a guy wouldn’t break up with a girl if she’s cheated?

I know he’s never cheated on her. :/