Barty was capable of hiding stuff - if someone bothered to provide him a good enough reason before he went and did something revealing it. But he did a lot of speaking and writing (in the journals) without really thinking. Really, the entry that Higgs had read to realize Barty was in a relationship hadn't been meant to announce it to the world. Evan had been jealous, and Barty didn't think he had a reason to be. So he had written that. And then...some people started knowing.
But Barty began laughing - hard. He hadn't thought about Rab's gambling at the time. He had been far too busy, mainly with shagging Evan more than once a day. Hell there had been the week before, when they had hardly left Barty's flat at all and had been much better at fucking than even...eating or sleeping. Thoughts on other people hadn't really been there. His mouth had opened to say as much when Rab spoke again.
His jaw dropped, and he was so flabbergasted, he wasn't even able to get his hand up to block the coin, much less catch it. His head shook as he winced slightly at getting hit in the forehead. But he was mainly just staring at Rabastan. "Wedding?" he questioned, completely confused.