Does {marath, will of the wild}'s ability deal commander or regular damage. My brother is arguing the point of it saying "marath" in the text line, therefore commander damage, even though I showed him specifically the "must be combat damage" rule. Can I get the official ruling?
It's combat damage only. It's name being in the ability's text line doesn't mean anything, it's only specifying where the source of damage is coming from (itself).
Commander damage is only dealt as combat damage.
Here's a rule that helps specify that combat is dealt only by attacking/blocking creatures.
119.2a (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=119.2a): Damage may be dealt as a result of combat. Each attacking and blocking creature deals combat damage equal to its power during the combat damage step.
Show your friend the rulings of 119, and it's subsections. It's all about damage and its different types/how to process damage.