Need Advice ... Please help

My older brother really likes this girl, he (26) is crazy about her (23). I've known here since school (we were really close friends at one point but stuff happened and we're not really friends anymore)The thing is I know for a fact that she is gay.So do I tell him and risk him thinking I'm making it up because of the falling out we had -OR- do I allow him to find out for himself and risk him being pissed off because I already knew?