Should I spill the beans about the affair?

I suspected my boyfriend was cheating on me and I was correct. He confessed everything to me including the fact that the girl he cheated with was married. Her husband is still in the dark and they haven't gotten the guts to tell him yet. I am not sure if they will. Should I spill the beans? I was not the only one affected by this and I don't believe this man should be lied to any longer. Am I stepping over the line?