"he is not a leaver"
Then what's the request all about? If he didnt leave but disconnected he should provide the pertinent screenshot, without it there won't be unban.
About the comparison you made: The user you reported left when two of his three lanes were down and sentinel was literally destroying the throne that's why he only recieved 3 days ban but MarkoMD left when only one of the three lanes were down and furthermore he left moments after he was killed what is kind of rage leave whereas froger.o_O left while he was alive.
All these little things are very important to determine an accurate ban.
But the main thing is that rage leaving is unacceptable thus I think 25 days of ban is a fitting period for this case.
Denied.