Help me help my friend?

So my friend idk let's call him Von. Has been with a girl since he turned 18, so about 4-6 months. And they are sexually active, he's IN LOVE WITH HER. And she kinda doesn't care. Like she's about 5 years older and " non-monogamous." But he states that she wants to have an open marriage and he's not okay with that. To the point that he went out and bought a ring last night and decided hes gonna ask her to marry him, I'm afraid shes gonna say no and break his heart. What advice do I give him to help him through this and make the right choice...

I forgot to add that he said " she will change after we get married, she won't have people on the side anymore I trust her." I dont think she will...