I have recorded clips of the achievements coming through whilst in lobby and I’ve just been onto theatre mode and recorded the suspect using the mod menu, you are able to see when me and my friend join the match about 20 seconds later.
The modder seems to be bridgemyster v2 as he’s using a mod menu at the start of the game before me and my friend even join, using aimbot etc. Shortly after spawning in I received a spam of achievements and as soon as I informed my friend he left so I assume he didn’t receive them. Me and my friend went onto Black Ops 2 and the first Team Deathmatch we load into is modded.