Benjamin agreed that it was lovely, and then smiled at Tristan, thinking about how lovely she was. He was so wrapped up in this thought that when Tristan complimented him he was momentarily flummoxed, rather stupidly thinking that she must have heard his thoughts, or he must have something out loud without realising it, and so she was responding. By the time he realised this wasn't the case, she had carried on.
"Oh, it's my pleasure," he replied honestly. "I'd only be playing gooseberry to my sister and her boyfriend otherwise. I'd much rather be with you."
That didn't sound all that complimentary, now he came to think about it.
Benjamin cleared his throat and said, "You look beautiful."