I wouldn't tell the boyfriend you don't like his friend. This leaves you at a tactical disadvantage because you've exposed a weakness. Ultimately your boyfriend will tell his friend your feelings about her if there's some perceived benefit in his mind.

Always remember, women are after emotional fulfillment and quality in a relationship whereas men are after physical fulfillment and quantity.

tl;dr If he can fuck her and you at the same time, he will. Men are experts at playing both ends against the middle. You're all just trophies or achievements to him.