Idk what to do

My bf ex text him thanks for being there i replied this is his gf she immediately told me i dont pay his bill amd I shouldn't awnser his phone cause im not his girl and that he is cheating on me so hes not my man she told me she going to keep cheating with him i asked her how long she said years, weve been together for 1 year so i dont get that i tried to get answers out of her but she wouldnt take my calls and wouldnt say anything else but goodnight hes a cheater but anyway I immediatly told him i was done i started hitting him and crying she just kept texting your mans a cheater he claims she texted that cause shes mad that he dont talk to her and then texted her to stop lien and a buncha stuff but idk whether or not believe him, i need serious advice