Your friend's DH sounds like he's taking money out of the market when it's down, doing something like day trading, buying individual shares or something else unwise if you don't know what you're doing. Or it could also be that he may have lost £10k+ a couple of times, but he may have made far more than that dozens of times.
You're almost certainly not going to lose money in the long term if you put money in a global index tracker in an ISA or pension and also have cash available so you never need to take money out of your investment when the market is down. Investing is volatile, but over time, the gains outweigh the losses, this has always been the case, even with the big crashes, the pandemic, current political instability. Also if you drip feed money in, the money you put in when the market is down buys more 'fund' than it does when the market is up, so more scope to grow. Read about pound cost averaging and 'time in the market vs timing the market'.
When thinking about saving for retirement, whether in a pension or ISA, the Meaningful Money podcast, which is by an IFA who tells most people they don't need an IFA, he talks about the cashflow ladder, which is (after you have an emergency fund and savings for things like holidays, car repairs, Christmas etc) putting all your money in investments, but looking at the money you know you'll need over the next 2-3 years max and moving that to cash. If you always think ahead like that, every 6 or 12 months when you look at taking some money from your pension or investments, if the market is down, you can choose to leave the money invested and look again in 3-6 months, when things might look better and just use the cash you already have - because you have a couple of years worth of cash, you can wait and things will almost certainly have improved in under a year. Also, if you have more than one type of fund, one might have performed better, so you could take money from a different one instead.
As for spending vs saving (or investing) the financial flow chart sets out a 'to do list' for money so you can see what needs doing, and yes, in your 50s depending on what your pensions look like, it could be the time to allow yourself to live a little, or you might be better putting as much as possible in a pension for the tax relief, you can also start to think about how long you need to continue working or if you can start to think about retiring or reducing your hours at work.
The UK Personal Finance Flowchart - UKPersonalFinance Wiki