My brother is being accused of kissing his friends girlfriend how can he persuade his friend its not tru!!!!!?

the couple are obviously having trouble because they are broken up with a week or so but i dont think my brother knew this but any way all their friends do not like her and think she like to stir a alot of s$$t. but any way the girl swore on her brothers grave (which i think no one should do, she obviously doesnt respect any one including the dead) to her boyfrined that she met my brother, but all close friends and myself believe it couldnt of happend because my brother looks up to his frined and his now exgirlffriend plus all his frineds that were there in the nighclub did not see anything happen but his friend will not belive my brother or other friends!!!!