she needs to tell him.... thats cheating.. i know thats yur best friend but you need to tell her what she did was wrong... :( its ok the same thing happened to me the other day i just got cheated on :( ugh
to b honest tell her tht she needs to tell him because its not relly ur place to tell him unless he is a really close freind of urs...and if she says no then tell hey sumne has to tell him and if its not her its yu....the boyfreind is gona do wat he wants about it so idk wat to say about tht