How much damage should a Rogue lvl5/Monk lvl6 be able to do with unarmed strike in 5e?

One of the PC’s in my campaign is playing a 5th lvl rogue/6th lvl monk. he uses unarmed strikes but has 2 magic items to boost his damage a bit: a set of bracers that add 1d6 slashing and a ring that adds 1d4 green fire damage.

In last night’s session, he did 84 pts of damage against an abominable yeti. His attack action (attack + extra attack) netted him 16 pt bludgeoning dmg + 10 pts slashing/fir from magic items. He followed with a sneak attack for 19 dmg (unfortunately, this is my first campaign to run and I hadn’t played D&D since AD&D, so I let him use sneak attack with unarmed strike when we started wihtout knowing better and it seems unfair to take it away now that he’s getting up in levels) plus 4 points magic dmg. He then burned a ki point for flurry of blows in his bonus action for 2 attacks dealing 9 points (incl. magic dmg) in the first hit and a critical hit on the second one dealt 26 dmg total. That’s a grand total of 84 points for the monk in one round.

Did I mention he’s a ghostwise halfling? A few session ago he beat an adult copper dragon to death with his bare hands. By himself. (okay, the dragon was trapped in a cave where he couldn’t fly and I wasn’t rolling the best that night and the halfling made his save against the dragon’s breath weapons and frightful presence, but still!).

Aside from allowing the sneak atack, am I doing something wrong here or did he just know how to build a monk that approaches OP?

Only good thing about this is that the ring he wears for the green fire damage is cursed. When he deals out a total of 125 points of green fire damage it’s going to explode, dealing him an amount of damage between 50% and 100% of the accumulated dmg. It happened once already with a lower damage trigger, but he didn’t get it taken care of, he decided to keep using it anyway, so…..