A tough question, one we all wrestle with.
My first attempt would go like this:
Perhaps you could find an issue on which you and your friend are both in agreement which is also the teaching of the Church. That way you are starting on a positive note. Then go into something you did, or did not do, in your past, which opposed the Church, and in that situation you should not have received communion and how you learned about your mistake later, or “came around” to accepting your decision was contrary to the Church.
Your friend already knows she is going against the Church by using the pill, and in my case, I can find many instances when I was the sinner. Using my own story would be a way of showing my friend that I do not think I am better than she is.
Then end on a positive note. Maybe remind your friend about a funny incident you shared when you were younger, something religious in nature.
If your friend ever wants to talk very directly about this, be honest but humble. Again, as you have already made clear, you don’t want to be judgemental. Then, leave it be-not just to preserve the friendship, but so you can be there for her as a silent example of both following the teachings of the Church, and an example of “love the sinner, hate the sin”.