Yes you're doing the right thing for your emotional well being. Tis better to hurt all at once and heal then to keep picking at the wound and it get infected. Your only mistake was getting involved with her in the first place. Learn from this and stay away from women who are already in a relationship and looking to cheat and have emotional or physical affairs. Same with women and men that are already involved with someone. They're confused people and the last person you want to be with when it comes to romance is someone that is CONFUSED.
Don't hang around waiting for her. Even if she does break up with him, she's still going to be "confused" and that means if he still wants her, he'll be doing to you what you did to him... which is trying his best to steal her away. If on the off chance she does leave him. Make sure she isn't still in contact with him in ANY way before you think you've "won."
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion