I just let you know that using this trainer is always on your own risk as the game has no real singleplayer. You'll notice it when you run the game. It has two options, find a match and host a custom game.
If you can host a custom game while being in offline mode, nothing should happen at all. But as I said, I've decided to make a clear note that the risk is on your own side.
//edit: Because your PM box is full, I'll answer here: If the game does NOT have VAC which would be declared on the description page, you'll not be banned by Valve. However I haven't developed the game and some developers care more about cheaters in SINGLEPLAYER than other developers. You should send them an email and ask them for a clear answer.
[Edited by 0x90, 7/17/2013 1:25:20 PM]
The description lists this as a single player or coop game. I played the same map in the Custom(single player) online and offline and both times the players generated where exactly the same with exactly the same gameplay. This sure appears to me that all the Custom game maps are strictly a single player only prewritten levels.
[Edited by thegrube, 7/17/2013 7:26:34 PM]