Can a Shimano 7 spd rear derailleur work with a Campy Athena down tube shifter? Late 1980s vintage Italian steel bike with Campy Athena mech (7 spd rear derailleur). Need to replace rear derailleur (because of weak spring not taking up chain slack in small ring). My thinking is to replace the rear mech with a Shimano 7 spd cassette and chain as well as a Shimano 7 spd rear derailleur (Shimano Tourney RD-A070). Will the existing Campy down tube shifter (which has both index and friction mode) work with the Shimano 7 spd rear derailleur (Shimano Tourney RD-A070)? Or do I need to replace it? And If so, what are the options?

If you are comfortable operating in friction mode, your existing lever should be fine.

IMO friction mode with a 7-speed cassette is still reasonable. I wouldn't want to use e.g., a 12-speed cassette with a friction shifter.