I need help about a frend?

well i have this frend. he's a rly nice guy and hes kinda goin out with a girl that will just hurt him. they've dated before last year, but she cheated on him n its just broke him. n i dont want that to happen again but he's goin back out wtih her. he sed that it wont happen again but i just dont want him to get hurt. n that girl that he's datin is my frend too. but the dude is closer to me and i just dont wnat to c him get hurt. how am i suppose to tell him this? n we use to date but we broke up. n if i tell him this he just might think im jealous or soemthing. but im rly not. he's such a great guy and i dont wnat him to fall and get hurt. my other frend feels the same way. how am i suppose to tell him???