How to kiss a boy if there is rumors saying he has snogged your best friend?

I am 13 and i was goin out with a real nice boy but i dumped him because i liked his best m8 and nah me and his best m8 are goin out but there are rumors that he went into the p.e cuboard alone with ma best m8 and they snogged and he tried to shag her and ma best m8 told me and i dont know who to believe but ma best m and boyfriend say its either one or the other and i cant pick but i really wanna kiss this boy but i cant if he has snogged ma best m8 some1 plz help ASAP!