WeMod using on coop

Have seen users play WeMod when playing StarCraft 2 how is it that they can use it on campaign and somehow circumvent it on multiplayer and coop I was matched to a user on coop together and he was able to spam Zerg with no supply and no resources cost yet get every thing out within 5 mins and somehow be in god mode no damage no losses and this was on brutal + I asked him how he did it and he said use WeMod is that that legal if so can anyone use it ?