I agree with @Coffeaddict . The fact that they are discussing it rather than just letting it happen is not a bad sign.
Discuss consent, safe sex and that you believe sex should be delayed until after he is older (he will agree, but you have very limited influence on what actually happens). Also discuss sharing intimate images, it never hurts to repeat that.
From my own experience with DD, you have very limited influence on this and what they decide to do. If you don't allow them to spend time alone in your home, they will find another place if they really want to do it. A "success" in our case looks like safe and consensual sex, not absence of it, even if I think both of them are too young for it, emotionally.
No, I would not discuss it with her mother except, maybe, in the most general terms.