I saw my brother's girlfriend cheating. What should I do?

Last night while I was busy strolling inside the mall, I saw a familiar figure sitting right beside a tall guy. I was surprised when I realized that it was my brotherís girlfriend. Weíre close and I know sheíll be happy if Iíll tell her that my brother will come home soon after his trip to a foreign country. I was about to approach and tell her the good news, but to my dismay I didnít bother anymore. Instead, I went home and thought about telling my brother that his girlfriend is cheating on her. What I just witnessed in the mall last night was an act of betrayal and dishonesty. The nerve was kissing that tall guy beside her and were so sweet together. I canít afford to have a sister-in-law like her. How do I tell my brother about it? Should I wait Ďtil he comes home?