Might be a pretty big legal argument if he just copied Reddit backend functionality. Would probably have to redesign the front end from the ground up as well to make it clear it’s not just a rip off.
I don't remember the specific case at the moment, but a few years back I think Oracle was suing Google (or some mix of big companies) about Google replicating the Java api but with a complete from-scratch backend reimplementation. Google wasn't using private Oracle source code, just building a replacement that used the publicly published api. Google won the case, and it I remember right that established public api's as non copyright or something. Again, not a lawyer and someone else probably has the details better than I do.
But reddit is so much more than an API… and I can’t imagine anyone wants to deal with the user-generated-content hell of actually hosting that kind of platform.
Google could afford it. They're still in sore need of a good social network.
No need for a front end, he’s got the app, that’s the whole point
I don’t think cloning products is illegal, and can’t find any patents held by Reddit.