Man that sucks how that person wants items on during your matches! Items IMO=No skill battle.
Not to worry however! Well, since one of the items included are pokeballs, I may be able to help you here. If there are Assist Trophies included in your battles as well, then you're in luck! Well, first things first. Blue Flash, I suggest that you look at this as this may help you to use some pokemon/AT characters to your advantage:
http://www.smashboards.com/showthread.php?t=215127
Yeah this was from my "failure thread" of an item guide. Still, I think it could give you an edge with this "competitive" item battle you are having!
Finally! I can refer someone to my item guide for the first time! =D Sorry. I was just SOOO disappointed that this didn't get a LITTLE more attention than it should have as I did put a lot of effort into it. I even updated it too...
I know it's just items, but still, not a lot of people knew about these facts... It doesn't matter if you don't want to read it though. It's bound to die soon anyway...
Hmm... stages...stages... Well, actually, out of all the stages there are available in the game, I think you've made the wisest decision about which one to choose against Luigi because Final Destination is one of Luigi's worst stages to be on. This is because Luigi relies on his aerial game to rack up damage and it's easy for him to do so because his aerials are quick compared to most of the cast. But, because there are no platforms on FD, this will make it harder for Luigi to utilize his good aerial game. Another bad Luigi stage is Jungle Japes. This is because it's harder for Luigi to get early KO's with his ^B move: The Fire Jump Punch. Also, it'll be harder for him to KO lighter characters such as Kirby since the ceiling ceiling is SO HIGH. Stages you should avoid choosing when against Luigi are Battlefield, Smashville and Corneria. As I said before with Luigi being on FD, Battle field is technically the polar opposite of FD. This is where Luigi can utilize his good aerial game. Also, projectile spam is less effective against Luigi here too. Both of these reasons are because of the platforms that are present on this stage. For Smashville, the main stage is fairly closed in meaning that projectiles can't be used as effectively here like other characters would be able to on FD. Also, there is a platform to help Luigi further when it comes to avoiding projectiles and to help Luigi utilize his air game. On Corneria, watch out for ceiling KO's! Unlike Jungle Japes, the ceiling is much lower here! If Luigi uses his Fire Jump Punch (^B move.) on you and you are at a fairly high %, you'll die at lower %'s than what you're used to being KO'd at normally.
Well since you play Kirby, stages I recommend you play Luigi on are Jungle Japes, (For the same reason above) and probably any other good, flat, no platform stages. Pick the stages you feel comfortable with however because you don't want to play on a stage which annoys you constantly. (Eg. I HATE NPC because I can't get over how spacious it is. Therefore, I always think it's impossible to KO people here which is a weird thought I can't seem to get out of my head... >_>) I was going to suggest Rainbow Cruise too because that's another good Kirby stage. Buuuut I think that would only just compliment Luigi's good aerial game once the boat starts to sink. Unless if your friend doesn't have an aerial game at all, you could still take them here I guess. I know you didn't say that you wanted stage suggestions, but if you want to win against this person more easily, It couldn't hurt for to try those stages I suggested.
Hmm... How to help you with the actual match-up? Uhhh... I don't really know how I can help you here... I've never really fought a good Luigi player ever. PERIOD. But, I do know that Luigi's main strength is in his aerial game. So try to combat him more on the ground because I think that most of his ground moves IMO are like: "Meh". Also, Luigi has low range ground AND aerial attacks. IMO, I think that Kirby's Bair + other long range Kirby moves would be of a great assistance here. You could also bait that person to miss an attack and just punish them for it easily due to Luigi's low range.
Also Blue Flash, can you lighten me on the details of how this person makes his Luigi campy? I didn't think such a thing was possible with Luigi! TBH, I thought his Fireballs weren't that GREAT for a projectile...
Uhhh...*AHEM* Yeah this happens to me often on Wi-Fi. This happens either when I'm playing seriously OR when I just feel like being cheap by spamming Kirby's copy ability on people within range online. NOBODY INTERRUPTS ME CROUCHING UP AND DOWN OR TAUNTING WITH KIRBY! 
Uhh... Yeeeeaaaahhhh, just ignore me... >_>
