I read efficient markets theory at one point and lapped it up. It is such a simple and alluring model, that had the side benefit of making me feel smart. Oops.

You should be glad someone you care about is taking an interest in something at that age and using it to keep his mind sharp.

Unless he's trading actively it's unlikely he's hurting himself much EV-wise. He's actually probably better off than most people who invest with a financial advisor who charges outrageous rates. Maybe he's not beating a low fee passive portfolio but keeping his brain functioning is worth far more than any small EV loss.

If you think his allocations are way off, instead of trying to talk him out of his hobby, you could buy him a book on portfolio theory or something and suggest that he diversify through ETFs to supplement his stock picking.

also, this

if you talk him out of this hobby he's going to find a new, possibly much more expensive one. maybe he'll start playing high stakes poker.

Bingo

Tell him about bitcoin.

Not enough info. What is he trading? How much leverage? How diversified is he? What % of his net worth is stake if the S&P sells off 40%?

With that said, allocating more than ~20% of your portfolio to a single stock is pretty donkish. At least ensure that's not what he's doing.

Also, introduce him to SPY/QQQ and TLT. Index haters come at me bro.

It is his money and he sounds like an educated man capable of making his own decisions at this point (79 isn't 99 haha.) That said, you could suggest he look at joining a trading club (I think a buddy of mine's grandparents joined one and they seem to like the social aspect) or challenge him to try it "on paper" only for say 6-8 months and see how he does. good that you care but if he is educated and not inhibited mentally due to age etc then not much for you to do IMO.

I think the way to make him lower is risk is by motivating him to gambol with a small portion of his wealth. He should get a seperate account where he can speculate all he wants and a main account for a low risk boring strategy.

It would be a shame if he pisses away his retirement. Almost inevitable unless he knows very well what he is doing or takes small positions relative to his wealth.

Not true at all, he'd have to run really bad to "piss away his retirement" unless he's using leverage. Even if he's being irresponsible and is heavily concentrated in just a few stocks with no edge he's a big favorite to be just fine.

OP,

My grandad did this until he passed at 92. Fortunately, he didn't know how to trade online, and his broker (or 'advisor') allowed him limited freedom to make his own trades (of which he likely paid heavy commissions for). He continued to clip coupons and he rarely sold. We didn't discuss it. He eventually bought a laptop and played online poker every day instead of watching CNBC.

I would avoid trying to explain anything to him. Especially about beating markets, how he's likely to underperform, etc.

Perhaps you could focus your questions on why? What's his goal? What does he plan to do with his profits?

I love how this thread became a bunch of brags about how folks beat the market themselves.