What prevents you from saying to him, "I would love to go out to dinner, just me and you?"
Next time he proposes going out with her again, you can say, "I wish you would be a little more considerate toward how I feel when you want to do more things with her than with me, and when you make plans with her without my knowledge. I know I can't do as much as I used to, but I feel set aside and taken for granted. If you are going to invest more in your friendship with her, or become lovers with her, I need you to make extra effort to let me know I am still a part of your life and important to you."
It's very simple. You don't have to wrangle with this in silence. Get it out. That car ride, you could have said to him,"So, what was it you wanted us women to discuss on the way?" Why are you waiting for him to make all the moves?