I won't lie, I didn't read through your log entirely, and probably in full disclosure I should have, but I'm going to field the answer/situation I've personally encountered with this same problem of a player joining and crashing, or being disconnected right away, while other players have no problem. If possible, to confirm this, see if anyone else can get on beside your friend.
If your friend is the ONLY one who can't get on, then his playerdate is corrupt or he is in an "illegal" location (possibly saved in a block, possibly just corrupted garbage and the server has no idea where to place him).
- In my experience easiest way to fix this is to delete his playerdata file. (you will want to back it up, copy it etc in case this doesn't fix it, just in case)..
- navigate to your world folder and then the playerdata sub-folder. Depending on the age of your server, you'll see a bunch of id files (look like cd keys, long alpha-numeric file names.. Which one is his is a bit trickier.. If there's only the two of you and you don't care about your location/items, you can delete the two of them (again rename the extention or copy elsewhere for safety, I recommend backing up the entire server). If there are multiple players, you'll have to download them and use a tool like NBTExplorer to open them and see which player they belong to, then on the server delete that playerdata file. Be sure, of course the server is not running when doing this!
I don't do as much in forge/ftb as I do in spigot, and I know in spigot there are easy ways (permissions systems do it) to lookup a players UUID. If there is a mod for forge that does this, there may be a command as easy as /lookup <playername> or something to do that - FTB experts will have to chime in on that..
This being said.... if you have other players, or rather you are the ONLY one who can get on the server out of more than just the two of you, it's not likely this will solve your problem.
