I Love You, But I Don't Agree with You

I have a friend who is in the process of divorcing her husband after 10 years of marriage. She told him six months ago that she wanted a divorce. I have listen to her complain, make light of the situation, and start making moves as if she is single. She is bringing up that he cheated 9 years ago. I told her that if they are still together that means that she forgave him and she need to stop bringing that up. We got into into it because I always voice my opinion. I'm not the friend that will just agree with you. If I think you are wrong regardless of the situation I will tell you because you need to make sure you have evaluated all aspects before making a huge decision. Am I wrong? Do I just need to keep my opinions to myself and let her vent? As a friend can I play devil advocate? I told her that I truly do love her but I don't agree with the situation.