Best friend got cheated by his gf. Don't know how to make him normal.

I 21M .So my best friend (22M)...one of the loyal and caring man,I had ever met. One day we're going out after our sem. over after couple of months. We reached Assi Ghat( varanasi),we sat there order chai and maggi and started enjoying view. While doing so I saw a girl with a guy sitting together The boy placed his hand on her waist. For a moment it appears, she is his gf but I just ignored as it may be someone other but I again confirmed. I made him saw that if he can recognised her. He just freezed, didn't say a word and we came back to our room. From yesterday he looks very sad..although he was very talkative but hadn't talk much from that day. How should I make him back to normal. Please suggest. I can't see him like that.