Saying 'Congrats' Would be Hypocritical

Many people believe that they are obligated to say the truth at any cost. I don't subscribe to this philosophy. The rule of thumb is: say what's on your mind if it will be helpful, not harmful.

In this case, I can't say how saying what you think will help at all. She knows all about his flaws, better than you do. There's is nothing that you are going to say that she doesn't already know. She's made up her mind and your words of caution surely won't change her plans. The only thing your words may do is harm your friendship with her.  

I understand that saying congratulations would be hypocritical. So you can simply say, "Good luck. I wish you the best." These comments are supportive and certainly true to what you feel.