Is he cheating?

If a woman said she was cheating your man and your man denied it who would you believe? This situation occurred an ex texted his phone got mad i responded then started telling me hes a cheater and i wanted to know what was going on i wanted answers but thats all she said i texted her from my phone and his she wouldn't respond my bf told her dont text or call hos phone nomo lieibg she laughed saying ok girl thinking it was me texting her but since then she hasnt hit him or me up and like im wtf! Why say your cheating and not wanna argue or semd screen shots she even said she knew about me she just said were cheating hes not my boyfriend and that was basically it