Leaving a run is not something you can report because it is not against the rules, with rare exceptions that cross the line to griefing. Leaving enough in Mythic+ can cause someone to be penalized by that is auto tracked by Blizzard as far as I know.
For other activities the penalty for leaving or being kicked is a debuff keeping them from joining another run for 30 minutes.
It is not fun to have another player leave a run, but it is not against the rules or something you can report.