Your Lady Friend is right, in most cases and for the biggest structures the triangle wins (mountains, hello), or even down on an atomic level (hello diamond, which is 3D triangles basically), but the closest to a circle shape that is easily stackable is the hexagon. And since they tried to replace hoses glued to the sole...
Smart girl, she is right. Look at the structure of roofing, triangles for that purpose. Honeycombs are used for maximizing volume (or area in 2d) to parameter, that's why bees use it.
