Could He Be Cheating?

One of my good freinds is having guy porblems and her man appears to be cheating....she found out that he was talking to another girl but of course he said that they are just friends and it could be but when she asked him for the number so they could talk he said no! and she got it from his phone somehow and he said that if she called that number he would tell her entire family everything and anything that could incriminate her....i tell her shes dumb and when we tried calling the number the girl didnt answer but when she called from his phone she answered and when my friend talked the girl hung up....could it be hes cheating or just trying to be a jacka$$ and make her feel bad?i don't know what more to tell her they have a really bad relationship and im at a complete loss as to way they are still together.....i dont know what more to say to her i tell her he is cheating and i tell her that she needs to let him go and that hes wating her time...but she's whipped....what should i say?