A Tubescreamer would work great with a twin. Sorry if I am reading a misunderstanding where there is none, but what your friend might have meant is that an overdrive pedal, like a Tubescreamer, would normally go between your guitar and the guitar input on the amp. The effects loop is generally used for other effects, like delay, phaser, etc.

Sorry, can't help with a specific Eagles style of overdrive, but you could get some great SRV with a TS & a Twin :)