My understanding is that no matter how much the opponent boosts their evasion/lowers your Pokémon’s accuracy, if your Pokémon’s ability is No Guard, then all of your moves will hit, no matter what. Sand Veil technically increases evasion (though it’s calculated as a percentage) so Dynamic Punch (a move almost all Machamps have) or really any other move, will always hit.
Note that you can also take advantage of No Guard if you have a below 100% accurate move that can deal a good amount of damage to the Machamp.
Also, so this isn’t confusing, I’m almost 100% sure that your cousin has a No Guard Machamp to counter Sand Veil.
Hope this helps! :)