I'm going to talk to a friends girlfriend
What do you plan on discussing with your friends girlfriend?
I think she needs to know the truth.
Are you sure you want to interfere in your friends relationship by talking to their girlfriend? It would be better if you just have a talk with your friend about being more honest with his girlfriend. You may ruin your friendship if you go to his girlfriend directly.
I think they are going to get married in the future and I'm worried she will back out of it after she finds out the truth.
Well being honest is always the best policy. Just feel you should talk to your friend instead of their girlfriend so they know it is coming right from you.