XFCE isn’t as light as people think it is. And KDE isn’t as heavy as it used to be.
Here is a more recent one:
Generally on the GTK side, I think MATE is probably the lightest of the more popular desktops.
But if someone is fine for bare bone minimal, LxQt and Enlightment are the lightest, but there won’t be much stuff there and support would be limited.
@haddy generally, for new linux users Linux Mint is probably the easiest choice as Mint is based on Ubuntu which is the most popular. And Mint itself is pretty popular among new users. For lightness, I would opt for Linux Mint MATE.
But before anything, try it on a Live USB.