If he was using the starters, then no, Yahoo doesn't frown upon it, but it's an extremely cheap way to attempt to win. It's not the way the game was designed to be played and the league should adopt rules to prevent that from happening. A max innings cap and/or limited roster moves would do the trick.
*Note--Rules changes should only happen in the offseason though. Too late for changes this year.
If he was just picking the guys up and dropping them without using them in order to keep them off everyone else's teams, then yes, Yahoo does frown upon that and can revoke his fantasy league privilidges. Just report him directly to Yahoo and they will take care of the rest.
Yes doctor, I am sick.
Sick of those who are spineless.
Sick of those who feel self-entitled.
Sick of those who are hypocrites.
Yes doctor, an army is forming.
Yes doctor, there will be a war.
Yes doctor, there will be blood.....