Only pass abuse report ID to AbuseReportMailer.

app/controllers/abuse_reports_controller.rb

@@ -9,11 +9,12 @@ class AbuseReportsController < ApplicationController
     @abuse_report.reporter = current_user
 
     if @abuse_report.save
-      message = "Thank you for your report. A GitLab administrator will look into it shortly."
-      redirect_to root_path, notice: message
       if current_application_settings.admin_notification_email.present?
-        AbuseReportMailer.delay.notify(@abuse_report, current_application_settings.admin_notification_email)
+        AbuseReportMailer.delay.notify(@abuse_report.id)
       end
+
+      message = "Thank you for your report. A GitLab administrator will look into it shortly."
+      redirect_to root_path, notice: message
     else
       render :new
     end

app/mailers/abuse_report_mailer.rb

 class AbuseReportMailer < BaseMailer
+  include Gitlab::CurrentSettings
 
-  def notify(abuse_report, to_email)
-    @abuse_report = abuse_report
+  def notify(abuse_report_id)
+    @abuse_report = AbuseReport.find(abuse_report_id)
 
-    mail(to: to_email, subject: "[Gitlab] Abuse report filed for `#{@abuse_report.user.username}`")
+    mail(
+      to:       current_application_settings.admin_notification_email, 
+      subject:  "#{@abuse_report.user.name} (#{@abuse_report.user.username}) was reported for abuse"
+    )
   end
 end