The octet rule means that atoms need a total of 8 valence electrons to fill their valence shell. Hydrogen only needs two to fill its valence shell.
Explanation:
All atoms except for hydrogen can hold a total of 8 valence electrons in their outer energy levels (2 in the s level, 6 in the p level). Hydrogen only has two electrons total, so it does not require more than the s level and two electrons to have a full valence.
The statement that the friend made is not true. most of the mass of the plant is from carbon. the carbon comes from carbon dioxide which is used during photosynthesis. the left over carbon from photosynthesis is used to to help the plant gain mass. there is a process for this which is called cellular respiration