Heart Matters: Whv did he betray me with my best friend?

Friends, your help is needed on this; kindly advice her, you may not disclose your identity just like the sender.

Her story....

I’ve been with the father of my three children for 13 year, but we only got married three years ago. I love my husband and I thought that after so long together, he would be happy to settle down for good. He has cheated on me several times in the past and I thought he would have sown all his wild oats by getting married.

I was therefore shocked to discover recently that he has been receiving and sending text messages, and making calls on his mobile to my best friend who was also my chief bridesmaid.
When I confronted my husband, he admitted that there was an attraction between him and her, but that they never took things further. I’m so angry and upset that I feel physically sick. I’m at a loss as to what to do.