just tell her she's better off and cheaters suck. tell her she's lucky she didn't stick around and marry him & have kids etc. if he cheats then, that's when the fun really begins. ask her what percentage of young people dating end up being cheated on. tell her it's like 85% even though it's just talking out of your ass. then say, "and that's how we identify who is good enough for you. i'll always be here for you no matter what." place your hand on her shoulder and give her a slight rub and then a pat. say, "you'll be fine!" and smile. look her in the eyes until she smiles with you. go get icecream.