Your neighbor is planning on investing money saved for retirement in a scheme you know will lose all the money. Do you stand by and do nothing?

I'd say no, of course not. You tell them it's a bad move. You don't give them a lift to the bank so they can withdraw the money.

I wouldn't advocate breaking their arms so they're unable to sign the paper work but there has to be things you can do short of watching them ruin their lives (or actively helping them), right?