If I had to hazard a guess?
They jumped in on Madness with someone else's lockout. If I remember correctly, the first person that goes in to the instance is can supersede the other people. Or like this....
Player A has a lockout to Madness from Guild X
Player A goes and joins raid group with Guild Z
Player A goes into instance.
The rest of Guild Z goes into instance, and is offered choice to take the lockout of Player A, which is 7/8H
Guild Z defeats Madness of Deathwing..... gets credit for 7/8H with no kill for heroic Spine because they did not actually get a kill as a raid for that boss (requires 8 minimum guild members if I remember correctly)
That is how that could happen. They didn't steal the other guild's lockout. They poached a lockout that was AT Madness off of someone, possibly a member from that other guild, hence the "theft".
I had this happen with one of my raiders back in .... t11. Our healer took her lockout to Nefarian to another guild and that guild went straight to Nef, got the kill, but we did the work of clearing all the bosses leading up to Nef.