SHOULD I TELL MY BROTHER THAT HIS GIRLFRIEND IS CHEATING ON HIM?

My brother and his girlfriend have been in an on and off relationship for about five years. I became friends with my brother’s girlfriend and we became close in a short period of time. They first broke up three years of dating then my brother started seeing someone else. It was hard for my friend to move on but after she saw that he had moved on, she found someone else too. After a year, I discovered that they had been seeing each other and my friend told him that she dumped the boyfriend and he dumped the person he was seeing, my brother really dumped his girlfriend but my friend lied about dumping her boyfriend. My brother said a bunch of mean things to the lady, and she was there for him when he was going through a rough patch, she was there for him when he was broke and had nothing. He didn’t end things well with her and I feel bad because he thinks my friend, his first girlfriend is being loyal to him. She has a man who she’s proud of. She’s always posting him and blocks him from seeing her statuses, my friend is lying to my brother and I don’t know what to do, because he loves her very much and I don’t think she loves him anymore. He is financially stable and he even gave her a key to his house. I don’t know if she loves him or not, she’s cheating and lying to him and I don’t know if it’s in my place to tell him.