Would you tell the girlfriend?

My boyfriend's brother came by to talk to him. He hasn't been doing well mentally. They went to speak in my boyfriend's "man cave" but had the door cracked and I heard them talk about his brothers girlfriend and his brothers exact words were "I fucking can't stand her." If you met the girl she's so sweet and kind to him. My boyfriend asked why and his brother said he doesn't know. She's amazing and he's tried so hard to fall in love with her and it's not happening and the longer he's with her the more he resents her. That just sounds immature... Then my boyfriend started encouraging him to just dump her. She's such a sweet girl. I feel like I should say something so she isn't blind sided but also maybe I should mind my business