I'd also have to check to make sure there is not a coded lock on non-actives gaining anything. For instance, you can watch your inactive sims work, but they cannot gain work performance without AwesomeMod. Similar to comm-skilling of TS2.
As for trying to do it just to make Lardo lose weight? Forget it. The moment Lardo leaves your sight, he will revert.
Really? I had an athletic simmie that would train lardos at the gym while his wife would woohoo with his friends behind his back. She was getting back at him for having kids with Brandi Broke and some other girl I can't recall. Well, when you train simmies you also become fast friends and since she can call friends of the household, she would call up his trainees and woohoo them while he was away. Those ex-lardos came to the house as thin simmies.
Now, I was snapping back and forth from the house to the gym checking on them, cueing actions for the missus.
Doc, waylay lardos that come into the gym with an athletic sim and train 'em (you also become fast friends with 'em).