It depends on the license agreement.
The owner of the intellectual proeperty can decide who is allowed to do what with his property. This is the same as for any non-intellectual property. If someone would take your car and drive around the block, this surely wouldn't hurt you. Or what if he'd just get in and out again? Would that hurt you? Still you can forbid that someone to do it. Simply because your car is your property and you can do everything you want with it. You can smash it, burn it or do something even more senseless with it. It's your property. I don't think anyone of you would consider that to be unfair.
So why do so many people consider it unfair that the owner of intellectual property (which can be much more valuable than any non-intellectual property) can do what he wants with what belongs to him? The owner of some textures can lock them on his hard drive, he can even delete them. You should be rather thankful that he lets you use them. So don't ever complain that he limits the use.
To come back to the question. Unlike Noon said, the EULA doesn't forbid anything. If the EULA would be the foundation of the right to sue you, you couldn't sue software pirates because they never signed the EULA. The rule is that (with a few exceptions) you aren't allowed to do anything with the subject of the EULA (ie. the protected material that is licensed to the licensee). The EULA allows you to do certain things with the protected material. So the named mod could be sued if using the textures for another game wouldn't be covered by the EULA.
And as for that certain case: Do you think anyone who makes his material available for modification intends that they are used for a competiting game? It's simply not the same if you modify the game the textures are from than to use the textures in another game. Just imagine this case: Two games, game A with excellent graphics but rather poor gameplay and game B with mediocre graphics but a new ground braking gameplay. The makers of game A are generous enought to allow mods to use the in-game textures. Some smartass mod makers then think they could use the textures for game B and then advertise "the gameplay of game B with the textures of game A". Why should anyone play game A again when game B has the same graphics but with a better gameplay? When the makers of game A opened the textures for the mod makers, they inteded to improve their game and make it more attractive, not to make another game so attractive that you wouldn't play (or buy) the own product anymore. To me, this makes perfect sense.