HOW TO CONVINCE MY FRIEND?

my friend studying inhis college, is a catholic but the students in his college who are protestants and pentecosts are fighting with him regarding his belief . he is trying to be friend with them but they are not accepting him as friends . protestants and pentecostants arguing that catholics are not true christians . i tried to convince him that he is atru christian but he is worried . how i can i pacify . please suggest???????? . since i am a muslim i dont know much about christtainism but i want to convince him taht his prayers will be listened by his god . also i ftried to speak with his college mates who are protestants and pentecosts butthey said i smust not interfere in christians affairs . i am not worried about the way how they insulted me bcoz i also like jesus . also i think all are equal but my only concern is i want to convince my friend and make him a friend of the pentecosts and protestan christians . even i am ready to sacrifice my friendship if they accept my friens as one of his .