I can't tolerate my friends boyfriend because of his mental disabilities
What is the reason why you specifically do not feel you can tolerate him?
I have spent some time with him, he's not smart at all. He can't understand my jokes.
I get that it may be hard to deal with people who aren't like you, but you still have to learn how to be more considerate in including them more. They are human just like you and deserve to have fun and friends.
Maybe I should tell her that her mistake to date a guy like him.
Not tolerating someone because they have mental disabilities is very rude to do. This is something that the person can not control, so try and not allow yourself to form a perception of the person for this reason alone. Try to get to know the other parts of the person despite their disability.