The only way I can see it being of use outside is to use it as it is, until it falls apart, and then dispose of it.
You could always donate it or leave it on the kerbside, or strip it down and keep the leather etc. or bits that may be of some use eg. casters!
If you are determined to give it a try outside , perhaps make it weatherproof, including underneath. Best wishes.
Are you looking to reuse the parts of the sofa? I wouldn't concrete over it. Perhaps deconstruct it and use the leather and other parts for some other projects. You could make something like this out of the parts. https://www.hometalk.com/23167093/scrap-wood-table-with-leather-sling
Sheila: Please don't use it outside! Find a newlywed couple who may want or need it!
Unfortunately given the materials of the couch and weather conditions, this isn’t going to work as you hope. You can definitely build a mould and pour a concrete couch though, that’d be cool.