How would you react when someone showed you his underwear?

Today, I was talking with a male friend about the jeans he was wearing. Basically, they were too loose, because he bought them when he was overweight. Then he lost about 20 kilos, but he didn't buy new jeans. While he was explaining that to me he unbuttoned his belt to show how much 'space' he had in front of him and how broad the jeans were, thus showing a bit of his underwear. I was surprised by his actions (usually my male friends do not do such things) and I tried to behave as though IT was 'normal' and I hope IT went well. How would you behave in such a situation?

How would you react when someone showed you his underwear?
How would you react when someone showed you his underwear?
Post Opinion