Hmm, that’s strange, since I wouldn’t have seen any news of these tabards no longer working. If rep is stuck at friendly 5999/6000 that’s usually a sign that whatever it is your friend is trying to do to gain rep is no longer apliccable to them, but the tabards usually bring you all the way to exalted.
I’m not sure which dungeons your friend is running, but some of BC dungeons are widely accepted as the fastest way to grind to Exalted, specifically the dungeons around “The Eye” in Netherstorm. Botanicula is one of the fan-favourites, and I myself used that dungeon to grind out reps back during BfA. You can do the dungeon once on heroic and up to 10 times on normal per hour, but usually those 11 runs wield enough rep to bring you all the way to exalted. If you hit cap with Botanicula runs in one hour, switch over to the Mechanaar, it’s the second best in rep yield.
If your friend is trying to gain rep passively through SL dungeons, then maybe that’s why there’s no more rep gain, since the tabards work best in pre-Pandaria content.
Have your friend try the above with Botanicula, I’d be very surprised if that doesn’t work anymore and you can usually hit Exalted in an hour or two.
Shoutout if this doesn’t work, I’m sure we can figure this out somehow