S&P 500 a safe stock?

I’m still new to the whole stock market thing and one of my friends has been investing in things here and there.

They told me that the S&P is a 100% safe bet to invest money. Is this true? I understand 100% safe is impossible but relatively speaking would slowly buying more and more of S&P be safe to grow money or are there safer stocks to put money in for the long term?