marth vs. peach is a pretty even match still in my opinion. marth might have the advantage with his reach and double fair, but peach still has some good combos on marth. in my experience, the kid that i know that plays peach sounds similar to your friend (doesn't wd, shffl, dashdance...) but still has some success with peach against newbies because of dash attack and downsmash. what i would do is a lot of shieldgrabbing (pushing A while holding your shield) and punish. also, if a guy is a bad peach player, by bad i mean he abuses dsmash, dash attack, and radishes, you should still be able to find ways around him. don't just c-stick with marth either because that's no fun (and your friends will end up hating you, but it might get the job done against a new peach) i would just say learn all the "advanced" (hate that word) stuff and in time you'll be beating that peach consistently
Killing peaches ~
If you get the peach to ridiculously high percents, try a f-throw and follow up with a n-air. If they DI away, you should be able to land it before they can tech with a short hop; if they DI up, you can usually land it with a full-hop. Some higher percent kills can be obtained by b-throw to b-air, but that's a pretty ridiculous amount of percent.
If all else fails, U-tilts work.Abuse the up-tilt because it normally goes through Peach's down cstick. The trick to beating Peach is repeatedly hitting her into the air. Your range gives you a huge advantage and if Peach isn't on the ground then she can't do the down cstick or pull vegetables which are her best moves. Also, try to pick Yoshi Story or Battlefield and when her damages get above 10% throw her up and abuse the up-tilt. Otherwise through her forward and hope she doesn't DI so you can get a tip.
dont be afriad to Fthrow> dash cancelled Fsmash. Yes, it can easily be DI out of, but it is quite surprising. First hit of DB sets up perfectly for Utilt. Ftilt is also a decent choice in this match, as it comes out faster than Fsmash and can kill at higher percentages.
dont forget, tippered Dsmash kills Peach at less than 100% on FD ; )