Not together anymore, but what about "just friends"?

Ok so thare this boy who was totally cheeting on my sister by kissing and maybe sleeping with this other girl 'p.s. I dont know forshure if thay slep together or whatever' ok so he told this girl he loves her and shes like 20 something and this boy is 14 but my sister is like 16 or around thare. But this boy even lied to my sister earlyer about his age that he was like a year or 2 older..but she let that go and like amillion other things. Any way and like he tells this 20 something year old that he loves her... 'I know!' and she bales on him. Know my sissy just found out about all this and its like hella crazy! And know thare never getting back together as boyfriend & girlfriend ya know. Should she still be his friend I dont think she should but she wants to! And im like really pist att this 'boy' so should thay stay friends? And should I like totally bale on him?

My opinion is that you should decide. No one online should control your life. Good luck ... I hope things go well