My best friend's husband is cheating on her and I can't tell her

My best friend's husband was friends with my husband. They met through me. They currently just had a falling out. Her husband and my husband would go on man trips. During one of these man trips he cheated. My husband was the only one to know. My best friend found out through phone records about the night of cheating. My husband and me are the only ones that know about it. My problem is my husband told me that he is still cheating on her and that is part of the reasons he doesn't want to be friends with him anymore. I don't know how to tell her without throwing my husband under the bus. But I feel like she needs to know. She loves him so much and gives me so much but he just uses her. They have kids together and she is a stay at home mom. I feel he thinks he can do what he wants because he thinks she needs him. It is a messed up situation. He cheated on her two years ago and who knows if it happen before that. He likes attention from girls. My best friend has no trust in him now and he caused that.