George is a little anxious about being asked to be Ted's best man. He barely knows him and only really regards him as a golf chum. Firstly he is worried that Ted doesn't really have any friends and secondly he has no idea how he will get a whole speech out just on the basis of playing golf together once every three weeks. He has turned to Lynne for advice, who should be able to pick up on his concern. She encourages him to play on his natural charm and wit to get him through the day and uses an example to back up her argument.
George doesn't look too happy at the end. This is because Lynne has broken the unwritten and unspoken oath that they took. From what Lynne says, it appears that George and Lynne aren't even married! Lynne is married to someone else, presumably a friend of George's, who asked George to be best man. Since that fateful wedding day, George and Lynne have been having an elaborate affair. This does explain why we often don't see George and Lynne together. Lynne prefers to do most of her social outings on her own so not to arouse suspicion. Perhaps Lynne's husband doesn't live in the Wimbledon area, probably in North London, and Lynne tells him that she works elsewhere but in fact she spends that time living her double life in South West London. They were getting on fine pretending it wasn't there, but now Lynne has brought it up, almost flippantly. It truly is the elephant in the room with regards to their relationship.