If two persons were talking about what a third person (He’s Shia) said romantically to another person.
1) And one of these two persons laughs, at what was said, does it count as mocking a believer?

2) In case one did laugh at the person, is it a sin? Should he apologize from him? Or does repenting to Allah SWT suffice

1- If what he said is halal, then yes it would be haram to laugh at him if the way it was done is considered mocking him. But if it’s a neutral laugh (like he found it cute or whatever) then it’s ok

2- If one laughed in a mocking wayC then one must sincerely ask Allah for forgiveness. No need to tell the person if it could upset him.