I'd probably have to see his list to have a good idea of what he could do to improve. It also matters what Craftworld you are. It's possible your friend has a fine list and is mismanaging it. It's possible his list needs tweaks, or is utterly unsuited to dealing with yours.
How exactly he fails also matters here. Is his difficulty in destroying your Wave Serpents? Dealing with your Dark Reapers and Fire Prisms? Taking objectives? Reaching your fast units with his assaulters? Keeping away from your assaulters with his fast units?
If his issue is dealing with Wave Serpents, I'd recommend he look into weapons like Lascannons that deal 1d6 damage, rather than weapons that deal a flat 2 or 3 damage. "1d6 minus 1, minimum 1" averages to 2.5 damage. An Autocannon will only typically deal 1 damage per hit against a Serpent, and a Stormstrike missile deals 2. If his issue is dealing with your infantry, I'd recommend he look into weapons like Heavy Bolter and Assault Cannon, which specialize in dealing with light infantry, or Plasma guns which are strong against infantry with better saves like Dark Reapers or Fire Dragons.If his issue is dealing with your tanks in the back, Lascannons are a classic choice. Militarum Tempestus allies would also help. For example, he could take a Battalion of Astra Militarum with stormtroopers toting all kinds of specialized guns to crack open tanks and transports.
All of this depends on how the game actually goes. The new Craftworld Eldar are strong, but just posting a list and being like "how does my friend, whose list and difficulties I have not described, adjust his list to do better" doesn't actually paint enough of picture to allow me to give useful advice. I'd recommend your friend post here himself with his list and his own ideas of how his games went to get advice.
EDIT: Some other things to consider. If he has a buffing unit like Captain or Lieutenant, is he making sure it's in range to give rerolls to his units? If he has screening units, are they in the right spot to physically prevent the Fire Dragons from getting into range of the tanks? If he has vehicles or infantry with heavy weapons, are they deployed in a spot where they can shoot all game without moving? If he has vehicles like Rhinos, is he using them appropriately to block line of sight? Is he making use of the right stratagems and powers? Is he deploying with decent range and LoS to things he needs to get at or shoot? Does he move his units in a way that accomplishes a goal every turn? Do his units that spend points to be very good at a particular task (say, a Dreadnought with a DCCW) get a chance to perform that task, or are those points wasted? Does he check ranges and LoS during deployment? Does he analyze which of your units represent threats and focus fire on them first? Does he understand what his list does and can do, and what your list can do? Does he have a plan to win, and does he update that plan as things change? Does he make good use of cover and unit positioning? Does he make sure he charges his units in the right order, so that his tough Rhino soaks up overwatch before his less-tough close combat unit charges? Does he shoot in the right order, starting with units that have few or no choices of what to shoot at, then making his decisions with his long range units when he has more information?
Almost all of these are things that factor a lot into your success as a player without a change in list. The Space Marines codex is weaker than the CWE Codex, but it's not THAT much weaker. It's possible your friend has a bad list, but it's also possible that he's mismanaging it or you're just much better at using your stuff than he is. These are things to take into account. I run pretty competent lists of Craftworld Eldar that look a lot like yours (Dark Reapers, tanks, Fire Dragons in Wave Serpents) against a Space Marines playing friend of mine and I have lost probably more than 2/3rds of my games against him. It's not because of the list, it's because he's a very tenacious and capable player.