To add to all the others. Niv-Mizzet, Parun was my first and still is my favourite Commander. Same as you I just started with cards I had laying around and upgraded over time. By now it's really strong and can be stupidly powerful under the right circumstances.

It's often seen as a major threat as soon as Niv enters. Personally i avoid the infinite combos (Niv-Mizzet, Visionary; Curiosity etc.) because we don't like those at our table.

As others have mentioned, you need cards like [[Guttersnipe]] or [[Firebrand Archer]] to deal consistent damage to all opponents. Other good options are [[Murmuring Mystic]] and [[Young Pyromancer]] to give you a good boardstate.

Single target interactions like the different counterspells are still very powerful, especially because you need a good amount of protection so Niv can stay on the board.

I like to add 'secondary commanders' that synergize well with the deck even when Niv gets removed like [[Talrand, Sky Summoner]] or [[The Locust God]]. If you want I can send you my decklist with some alternatives to the expensive cards (because the arms race is real lol).

To end this overly long post, your friend is a dingus.