Investing in the above recommended index fund ETFs is usually pretty damn diverse. People have this inherent instinct that only having 1-3 "things" means they're not diverse, what they're not taking into account is those 1-3 things were designed to be diverse. Don't think of it as owning one index fund, think of it as owning little bits of hundreds of stocks. I know a surprising number of people who invest in every single fund in their 401k to be "diverse" and have no idea what any of the funds are. They just figure more funds = more diverse.
Are they trying to diversify for a situation where Samsung, Exxon, 3M, Goldman Sachs, and the rest of the 100s of publicly traded companies in their portfolio all fail at the same time? If so, I'd suggest a large garden and a water well, with maybe a couple of guns and some ammo.