Ok first let me say that Marth isn't a easy character to pick up and own with. So if you believe in your friend's words then you need to get that out of your head right now dude, its only mere discouragement. There are plenty things that need work with Marth spacing, edgeguarding, dashing afte aerials, I could go on and on but I digress.
Anyway, Ganon is a very tough match up for Marth simply because Ganon is a tank. Ganon, at low percents, can take many tippers from Marth better than Marth himself (assuming its a ditto). The trick for the entire match is to push Ganon towards the edge were Ganon will have limited choices and weaken him at the same time. Forward throw to down tilt, to grab at low percents is your best friend in this match. Just remember to stay spaced, and don't let the cheesy Marth is easy to play idea get in your head.
If your friend feels that way perhaps should challenge him to a Marth ditto and see how he does.