Filter Github issues closed by a user

You could list repository issues, filtering by state: closed.
The answer includes a closed_by field that you can use to filter the result set.

Another approach, using GitHub GraphQL, is possible en though the Issue object does not have a ClosedEvent field which actually has the Actor field with the user details.

GitHub support suggests in that thread

query commits{
  repository(owner: "rohit-smpx", name:"inno"){
    issues(first: 10, states:CLOSED){
      nodes{
        number
        title
        timelineItems(itemTypes:CLOSED_EVENT, last: 1){
          nodes{
            __typename
            ...on ClosedEvent{
              actor{
                login
              }
              createdAt
            }
          }
        }
      }
    }
  }
}