There's a difference in doing someone a small polite favor and doing unpaid labor. I definitely have previous employers to whom I'd give an hour of my time to sort out a quick issue, and for a lone hour it is not worth generating an invoice and dealing with taxes. At most I might accept a meal or a pint.

In my books doing a favour to the old coworker is much more worth than writing an invoice for one hour of “consulting”. They’ll be grateful and owe you a favour.

I totally agree. I'm sure there's a significant number of us that have landed wonderful, well-paid jobs thanks to former co-workers. That's totally worth a few hours of unpaid support for a former employer.

There's a difference between getting exploited and making a deposit in the favour bank

It may also cost you your current job if you take money.

Most employment contracts have stipulations about working for competitors. They're rarely enforced, but you can sometimes get caught out if something goes pear-shaped.

If you haven't taken any money, it's a lot harder to hang this on you.