No, it isn't.
Iron Valiant is in a class of Pokemon called Paradox Pokemon. These are Pokemon that have either Quark Drive or Protosynthesis abilities, but look like ancient / futuristic variants of other Pokemon.
However, they are not legendaries, for the main reason that they spawn, for want of a better word, renewably, meaning you can catch an infinite number of them, if you really wanted to.
In Valiant's case, it has Quark Drive, and is a futuristic version of Gallade and Gardevoir, and respawns when you catch (the static one) in Area Zero.
The only contrary evidence is that it is separate from the rest of the Paradoxes in the Pokedex. However, in my opinion, this is because Game Freak wanted to make Goldhengo the 1000th Pokemon, and a Paradox doesn't make as much sense there instead of a Pokemon that has 1000 in its Pokedex entry, and lore, so they split Roaring Moon and Iron Valiant from the rest.
So, no, your friend was wrong.
