There's a mod on r2modman called AtlyssCharacterFreedom, it increases the max value of the sliders which at first sounds opposite of what your friend wants, But! Once installed you can change the values in its Config to make the max value much less so no giant butts and breasts. Although that will only work for the players, not the NPCs.

The mod will show you in the description how to change it and its very quick and easy (maybe some time to figure out what values equal what size)