As a general rule, grabbing is one of the most important aspect of Marth. Grabs are a good Marth's trump card. Dash dance through an opponent's approach, grab them, and then combo with u-tilt, u-air, fair, and usually finish off with a f-smash or n-air. Being able to jump cancel grabs is also really important.
This isn't always the case, but grabbing is almost always the best course of action that you can take. Much smarter than throwing out dash attacks and f-smashes.
When you're on the ground, you should basically spam shffled fairs. Retreating, advancing, standing still, fair is always a great move. And, yes, tippers are very important. Spacing should come naturally with experience, when you start to get a good feel for Marth. Be sure to use dashdancing and wavedashing to your advantage, in order to score a lot of tippers.
Try not to underutilize any of Marth's more obscure attacks, either. d-tilt is a very safe approach, and you can cancel it early with another move. Dancing blade is also a great move to throw out every once in a while, it makes a great anti-spotdodge. You can use your d-air in many situations safetly, I love u-throw --> d-air on space animals, off the edge. You can also jump out and spike them down, if you do it right. It can also be used as a final-stock spike, but try not to overuse it, unless you're feeling really flashy. F-tilt is basically a mini-f-smash, and never forget about the reverse dolphin slash. People almost never see it coming
You can mix up your edgeguard with Shield breaker, d-tilt and counter. Depending on who you're facing. Shield breaker actually makes a really nice edgeguard move, especially against space animals, and captain falcon/gannondorf. Counter can be used situationaly, as an edgeguard, and d-tilt can easily poke them away, to set up for a more abusable recovery.
Basically, try not to fall to temptation, and just start throwing out F-smashes. Be sure to utilize your fair, almost as a shield, and use Marth's insane grab game to it's fullest. It'll take some practice, but if your friend is willing to learn, he'll definetly have a great time, Marth is an awesome character, but he sometimes get a bad reputation of being spammy. Try not to play that way, because we all know that Marth is so much more l337 than that.
That's a lot if info, but hey. As they say, when you're learning, information overloads are a good thing