Ex and hiding?

If your bfs ex told you his ex threatened to end her life because he left her for you and after 5 months of being with each other you found out from him that he had texted her after 5 months of endining it with her..is it okay? Especially if he hadn’t told you but you found out cuz she replied to his message...nothing flirty just saying him texting her useless especially after “you’ve made it clear you didn’t want anything to do with me” ... how would you feel