How do I politely tell my bf that his friend is using him? Or do I stay quiet?

Good evening! I have noticed this to be a tendency for a long time now but I tried to play it off as my boyfriend just thinks that his friend is going through a tough time, however i’m beginning to think he is being used and he’s making excuses to hide it from himself. This friend will randomly show up no notices given and talks to my bf nonstop for hours at a. Time about his problems. He never really lets my bf talk. He only wants to hang out with my bf when he has no one else to hang out with. I feel so bad for him considering this is the only guys friend he has that comes around. Unfortunately I think he has poor friend making skills lol any advice accepted.