Asking on behalf of a friend, how to get to FIRE starting at 30 or even 40 ?

Mr Money Moustache's article is a great starting point https://www.mrmoneymustache.com/2012/01/13/the-...

He essentially outlines that cutting your spending rate is much more powerful than increasing your income. You can refer to the table in the article linked above and see that if you have a savings rate of 50%, you can retire in 17 years. Save 70% of your income and you can retire in 8.5 years.

So if you earn $50,000 a year, save and invest $25,000 of it, then you can retire in 17 years. This means that after retirement, you need to continue to live on $25,000 a year. But with geoarbitrage, you can stretch your money way further.
Another way to approach is to figure out how much money you need when you retire and work backwards. If you need $40K a year to live on, you need to work towards $1 million.

It's not that crazy, honestly. If you save $3500 a month (you and your wife, if you're married?) at 6% for 15 years, you'll have a million bucks.

Spend less, earn more (increase topline salary & get a side hustle), save more, invest in dividend stocks and/or growth stocks. Rinse & repeat for 10-20 years.