Infidelity/Betrayal should i take her back? Believe her or not?

I broke up with my ex cuz she she went to a club without letting me know. I stayed home and she knew she never called me back just text to say goodnight. I got a text from a friend cuz she was seen talking to a guy hes 24 and she 17 me and him have problems and I know he likes her hes a playa type of guy. So heres the situation. She tells me he went up to her and said hi so she couldnt hear him so they decided to talk by the bathrooms . Later i found out the whole truth and even that they danced together. So she was dividing the truth til i saw pics. She said its just a friend but now I cant trust her. I talked to the guy and he said she went to talk to him and he like are you sure your boy won't trip. I talked to her and she swears it was just one song and he went up to her ... Im about to make my decision Friday we've been together for 1.5 years. Even if he did should I be able trust her?