He suggested perhaps buying long-term quite OTM (hence cheap) puts on a short-list of highly over-valued firms.
Any suggestions for such firms?

Always a tough call, but this list might offer some inspirations. I'm sure there is some fluffy hype in there somewhere.
greenstocknews.com - Artificial intelligence stocks

Note, when buying put options for profit, often you will want a combination.
Put options are expensive. But you rarely expect your target to drop 100%, so you might want to buy something that pays off with a drop to the interval of (say) -10% to -40%, maybe -50%.
That can be done by buying a put option that starts profiting when you want it to, but selling another put option (same ticker, same expiry) at a much lower strike.
This can notably reduce the up-front cash cost of opening the position, while still accomplishing the bulk of your investment goal.
Writing the put is not an exposure risk, PROVIDED you don't close the higher strike one first. If the firm goes bust, the one you've written will lose a whole lot, but the one you bought will make more.

Jim