[Swirrel]

Below addons work with Carbonite. Several of them have been around for years and years, where others are newer, but still works with Carbonite.
Adorable Hatchling Nests
Bagnon
Bartender4
Carbonite
Gatherer
HandyNotes
HandyNotes-AzerothsTopTunes
HandyNotes-DraenorTreasures
HandyNotes-HallowsEnd
HandyNotes-LunarFestival
HandyNotes-WorldMapButton
Master Plan
Mast Plan A
PetJournal Enchanced
PetTracker (and a gazillion of those starting with Titan Panel Rep/archeology/pet ect).
Quartz
Skada
Titan Panel (and a gazillion of those starting with Titan Panel Rep/archeology/pet ect).
WIM
Z-Perl
Deadly Boss Mods

There is really only 1 (imo) fail-safe way to check which addon-/s (if more than 1) is causing this issue. I'm afraid it's a little time consuming, but at the same time satisfying to know which is causing the issue.
And tell your friend that doing the set-up on Carbonite or any other addon is important. Perhaps more so with Carbonite, despite that it's been split in to severals addons. So more the need to set it up right.

First:
Get an addon that can track any errors/bugs, so if there are any, the error can be reported to the Author-/s and possibly fixed. I'm not entirely sure if Blizzard still has it's own feature for debugging. But either way, if an error notice pops up on the screen there's a chance it will tell you which addon/module is causing the error/bug.
As I said before, the listed addons works with Carbonite. However, you might want to disable HandyNotes in the beginning. (It can't hurt).
Important: Remember which addons you turned on last and second last. Write it down if you have too.
Turn Carbonite on and 2-3 other addons that your friend can't "live without". Log in. If the issue isn't thre, log out, turn another 2-3 addons on and log in. If the issue still isn't there, log out and turn on another 2-3 addons ect ect ect untill there is an issue.
If/when the issue arises. It's super important that you're able to remember which were the last 2-3 addons you turned on. So log out and turn all but 1 of the 2-3 off and log back in... Do you still have an issue?
If no, log out, turn 1 more of the 2-3 on and log back in, if no issues still, then just to be sure, log out and turn the last of the 2-3 on and log back in, if you get the error here, then write that addon off for now.
Continue with logging in/put for testing to see if there is more than 1 addon causing issues.
If there is, write it off for now as well.
When you're done, and if you have more than 1 addon causing the issue, either live with the annoying issue you have or
find another addon that does the same, but works with Carbonite! or
simply learn to live without the addons causing you issues.

Second:
Exit game and launcher.
Head in to the folder or drive where WoW is installed, navigate to 1: WTF > Account > "Account Name" > SavedVariables.
DO NOT delete anything you don't know what is!
Should there however be any names of an addon that you no longer use, delete it's 2 files. If there is more than 1 addon name in there that you no longer use the addon for, delete it's 2 files.
When done, close the window.
Log back in to WoW. Do you still have issues. If no. Grats you're done.
If yes, log out and go forward with option 3.

Third:
Close the game as well as the launcher.
If above steps doesn't work, and most of us probaly really dislike the next step is to make a copy of the 3 folders: Cache, Interface, WTF, then place them somewhere else for back up. then delete the original 3 (cache, interface, wtf) and start the game back up and log in.
Silly really, but turn debugging on and either reload or log out/in. Anything? If anything, report it to Blizzard that you're getting errors after a reset of the 3 folders and that you have no addons installed. Wait for reply, if any.
If no issues, exit game and launcher.
Start with downloaded all addons anew. Yes it's a pain, but sometimes necessary and it's best to do it on a clean "install" of the 3 folders.
Log in with ALL addons turned without "load out of date addons) turn debugging on. reload or log out/in. Anything? if no. Exit game, restart pc (because it's wise though it may not matter, but can't hurt), log back in to wow, anything? no. Continue setting up your addons.
Delete the copy you made of the Cache, Interface, WTF folders.

The Third is a bitch. But sometimes whether we like it or not, is necessary.


Figuring out which addon-/s is causing issue-/s is a pain in the ass, but it's well worth the effort AND you get to know which addons works together with the ones you just have to have.
When the problem is solved you can turn debugging or the debugging addon off again. It's no fun to have even the tiniest errors pop up in the middle of your screen while you're busy doing other stuff.

Also if you're friend aren't up for the way to troubleshoot addons, perhaps said friend shouldn't use addons or at least not as many to begin with.
It's not to be mean, but asking for help and not really being interested when it requires a little more than the most simple steps is just not OK.

Well. Anyway, the above troubleshooting step I've provided is something I myself have used quite a few times for the past 10yrs I've played WoW.
I started with 200+ addons and not counting Blizzards own stuff. I started using Carbonite shortly after it was released and was only able to be downloaded from carboniteaddon.com for the newest version.
I've only kept the addons above that I myself at any time have ever used while also using Carbonite, aside from Bagnon. Several friends/guildies over time have used Bagnon and Carbonite at the same time and never reported issues to me with the two together (I talked them in to using Carbonite).

If your friend is having any other issues, see you're private messages.

//Swirrel

Ps. Attachment is just to show my personal current addons that I know works with Carbonite.