Opera is not to blame for high memory usage.
It's the websites and number of tabs and pages open that count.
More than likely the user does not use Ublock origin which blocks a great deal of website junk.
There is also the unresolved problem that nowdays websites are alive; they are constantly loading the background.
That means when once a website has loaded the user should execute a keyboard shortcut to mimic hit the X in the address, to halt content or kill the website. Technically, Opera, I believe, is at fault in this department as I believe they have a responsibility to help the end user out and therefore to kill webpages once they are loaded or after X amount of seconds; this feature is absent in All browsers including Opera. But is such a vital feature.