That's really sad. I can't imagine EVER telling my daughter she was no longer welcome in my home.
My mom's best friend was gay, and had a real hard time. She is really sensitive about making sure that we know we have her unconditonal love. My dad was once teasing my sis about not having a boyfriend, and so she jokingly said she was gay. My mom spent the whole day hugging her and telling her she could be gay if she wanted, and she loved her no matter who she loved, and she could always bring her girlfriend to the house...my sister was like "WTF?"
It was really funny. It made me feel good to know that my mom would react that way should any of us "come out" for real.
I hope your friend's mom comes around, but in the meantime, it's so great that YOU are there for him.