I have experienced a lot of what you're talking about. It's a strange place to be in - there's a lot of mixed emotions, or there was for me, when my partner started having problems with his gf. On the one hand, I was sad for him, and wished he didn't have to go through the pain of it, on the other hand, I was slightly relieved that, at least for a while, the complications and challenge of another person would disappear for a while, and on a ...uh... third hand, I was stressed too because I liked her and didn't want her to leave either!
You're right though, there's nothing much to do except be present, in whatever way works for you both, and let him mourn her. It might be shitty to see him visibly express the degree of his love for her, but supporting him through it could deepen your relationship, and you can learn a lot about yourself, and him, through this break up.
Good luck
Hoping you can help heal his heart