I saw my best friend’s ex, that left him because she was “lesbian”, with another dude. Should I tell him?

My best friend has been down for the past few weeks, because of this girl he was really passionately in love with, she left him because she was “lying to herself about being straight to fit in”. And it’s taken a toll on him, his well-being and his grades. Today I saw the same girl with another of my friends, another dude I’m close with, I was really confused because I thought she was a Lesbian, apparently not...

Should I tell my best friend risking hurting him and my other friend, or keep my mouth shut?