He’s a good friend for sticking with her regardless of her disinterest in a relationship with him. Remind him of that. That shows commitment and real compassion and care. When he says “I’m used to it by now”, he means he’s had unrequited feelings before this experience. Like I said, that’s admirable. Some people only love people who can love them in return. He’s a giver, which is a beautiful thing to be. Just keep assuring him that he’ll find someone who feels the same way.