There is always a crash coming. Historicly it generally takes around a year to recover from a crash and then the market continues to grow larger than it was pre crash. Google to see the graphs of world trackers over the past hundred years, see for yourself. Find the dip that happened because of covid for example, see how it went back up again.
He has 10 to 15 years before he retires, that's a good amount of time to ride out the bumps in the market.
You say he's hand to mouth? How much can he afford to put in a pension monthly? Is he aware of pensions tax relief? If he puts in £80, the gov puts in £20
Use a compound interest calculator online and plug in some numbers. For example:
If he puts in £240 a month for 15 years and the gov puts in £60, that's a total of £300 monthly, if it grows at 8% (rough historic average of global/US tracker) he could have a pot of £100k on retirement which is (sadly) above the national average, and goes to show there are a lot of people in his position.
Of course he could invest in funds which are less volatile than a world tracker, he could put a percentage of his pension in 'safer' funds such as bonds, but then there's less potential for growth.
Doing something is better than doing nothing.
You asked for an online course - Rebel finance school on YouTube is always recommended on here to help with financial literacy. It's a bit exuberant in tone, you'd asked for something calm, so I don't know if it will suit, but the information is solid.